Latest Offer: 200GB of data for just £23 a month!
EE’s best selling plan is a 5G ready 200GB data plan with unlimited texts & minutes for just £23. It’s a 24-month basic plan. It may not contain any extras, but you’ll be getting the most 4G and 5G coverage, as well as fastest 4G data speeds in the UK!
Who is EE?
EE is the largest mobile network operator in the UK and it’s one of the four major network carriers in the country. The network officially launched back in 2012 with the merger between T-Mobile and Orange, and not too long after EE became the largest operator in the UK. In 2015 BT bought the mobile provider. EE is the fastest 4G mobile network in the UK and its 4G coverage reaches 90% of UK geography and over 99% of the population.
EE is committed to constantly improving its network and currently offers the fastest 4G data speeds, as well as the best network coverage for both 4G and 5G. However, the best coverage and fastest speeds come at a higher price point.
- Fastest 4G data speeds and best network coverage in the UK.
- Largest 5G coverage in the country.
- Allows data sharing up to 100GB.
- Some tariffs are considerably pricier than other mobile providers’.
- International roaming is very limited compared to its rivals.
Who is O2?
O2’s has always been known for quality when it comes to the UK mobile network market, but that reputation also means customers can expect to pay slightly more.
O2 is the second largest mobile network operator in the UK and offers great features & extras you can’t find in any other operator. An example would be O2 Priority, which offers customers discounts and freebies from well-known retailers & exclusive early access to purchase tickets for gigs & concerts. O2 gets the basics right as well with over 99% 4G population coverage, but they are tracking behind the other major mobile network operators when it comes to landmass coverage and data speeds. O2 ranks second in both widest 5G coverage and fastest 5G data speeds in the UK.
- Priority App alone makes O2 a good choice.
- Over 16 thousand free Wi-Fi hotspots across the UK, plus O2 WiFi Extra!
- Flexible pay monthly plans. You may also build your own plan when buying a handset.
- Decent 5G coverage and data speeds.
- The slowest 4G internet speeds. Also not as great network coverage.
- Restricted unlimited data plans.
Table of Contents
Plans & Pricing
SIM Only Deals
EE has two different types of pay monthly SIM only deals to choose from. Lower-cost plans are called ‘Essential Plans’ and some of them include 5G coverage, however, they all offer up to 60Mbps 4G data speeds. Given how EE is unrivalled when it comes to 4G coverage and data speeds, you still get good value with Essential Plans.
EE Smart Plans all come with 5G coverage, and at least 1 Swappable Benefit. You can read our EE Swappable Benefits guide to learn all about them. There’s quite many!
Just like its competitor in this review, O2’s plans are on the pricier side, but for multiple good reasons. There’s only one type of SIM Only deal on O2, but the network offers some flexibility when it comes to the length of a contract: 1 month, 12 months and 18 months. Regardless of your choice all SIM Only plans come with unlimited minutes and texts (except 500MB and 1GB data plans if you select a 30 day contract). All O2 customers can enjoy daily offers from O2 Priority and O2’s thousands of free Wi-Fi hotspots.
The cheapest O2 plan is priced at £10/month and it locks you in for 18 months with the network. You only get 2GB data allowance with this plan, but you also get to enjoy 6 months of Disney+ which adds more value to this tariff. If you wish to stay on a monthly rolling plan 50GB is the largest data bundle available at the moment. If you want an unlimited data tariff you have to sign up for at least a year. If you’re interested we’ve done a detailed review on all of O2’s price plans.
Half Cooked’s Verdict
Both networks offer great value for each one of their plans, but O2 offers more alternatives and better extras. On top of this, O2’s pricing is a bit more wallet friendly when compared to EE’s deals.
Pay As You Go
At the moment there are two ways to join EE’s network on a Pay As You Go basis. In addition to these two options, you can also use EE’s PAYG services in a traditional sense at 35/p minute and 15p/text. While you can go old school with EE, the network won’t let you use any data, which means you won’t be able to connect to the internet.
To use EE’s network and access the internet you have to purchase a ‘pack’. Packs are 30-day bundles and they come with data allowances. With the exception of the 4GB data pack, all EE PAYG deals come with unlimited minutes & texts.
Alternatively you can purchase a Flex Plan, all EE flex plans come with data allowances. Both options are similarly priced, but all Flex deals offer a limited amount of minutes.
O2 proposes two types of Pay As You Go alternatives to its customers and these are called Big Bundles and UK & International Bundles. O2’s Big Bundles are a bit pricey and they offer significantly less data than EE’s Packs.
Unfortunately, you can’t use O2’s network like a traditional PAYG service, which means you need to top-up every month in order to keep your SIM active.
While both networks offer PAYG services, there are some other great alternatives such as giffgaff which is powered by O2. The cheapest PAYG deal in the UK is offered by 1pMobile with just 1p/minute, 1p/text and 1p/MB data and the network piggybacks off EE.
Half Cooked’s Verdict
EE provides a wider range of selection when it comes to PAYG deals. Plus, EE’s deals offer more data for less money. You can also use EE’s network on a traditional PAYG basis, while this isn’t possible on O2.
Pay Monthly Phones
Both EE and O2 have a wide range of mobile devices for you to choose from, including refurbished options. Both networks offer handsets from manufacturers such as Apple, Samsung, Huawei, Sony, LG and more.
Both networks also provide a warranty service. O2’s warranty lasts for a whole year, while EE’s ‘Service Pack’ feature offers a lifetime guarantee for your device at no extra cost. While this makes EE look like the clear winner, we have to mention O2 Refresh, which splits your bill into two parts: a phone plan and an airtime plan. Thanks to this feature it becomes easier to manage your bill on O2.
Half Cooked’s Verdict
Both networks offer similar devices for their pay monthly deals. EE’s ‘Service Pack’ feature offers lifetime warranty, while with O2 Refresh your overall bill is split into two: Phone plan & airtime plan. This gives you more flexibility and makes it easier to manage bills. We think you can’t go wrong with either network when it comes to pay monthly phone deals.
Network Coverage & Speeds
2G, 3G & 4G Coverage
When it comes to network coverage and data speeds, EE is the ultimate network in the UK. The carrier’s 2G, 3G and 4G population coverage is over 99% and it also provides over 90% geography coverage. The UK’s new emergency services network is being built on EE’s 4G network and the provider aims to cover even more landmass before this year ends.
All EE plans benefit from the fastest 4G data speeds in the country. With customers able to get up to 60Mbps 4G data speeds. If you’re on an EE Smart Plan you have the ability to get even faster data speeds but this depends on your location. While EE has the best coverage without a doubt, you should always check your frequent areas before joining a network.
O2’s 2G and 3G coverage are solid, but the network’s 4G coverage lags behind other major providers. Looking at Open Signal’s October 2020 report, we see that O2’s average 4G download and upload speeds are decent enough, but not as good as EE, Vodafone or Three. O2s’ 4G download speed was measured at 18.2Mbps, while its upload speed was at 6.2Mbps. EE ranks as the fastest 4G mobile network in the UK with 36.4Mbps average download speeds and 9.1Mbps upload speeds.
Before you order a SIM from O2, we strongly recommend you check out O2 network coverage in your frequent areas.
Want to learn more about 4G and which networks have it in the UK? Then head on and check out Half Cooked’s detailed 4G Networks in the UK guide.
Half Cooked’s Verdict
There’s no competition under this category. EE has the fastest data speeds as well as the best network coverage. O2 takes the last place among the four major operators in the UK.
This will be yet another one sided match-up as EE’s 5G coverage is still head and shoulders above O2’s. The latter’s 5G service was launched back in October 2019 and it has been improving steadily. Right now, O2 can offer 5G services in 100 cities and towns, meanwhile EE’s 5G covers 112 cities and towns. We must add that EE’s coverage is wider per location. According to OOKLA’s October 2020 speed test results O2’s 5G data speeds (176.9Mbps) are faster than EE’s (134.7Mbps), but it’s also reported that EE’s 5G network is more stable.
At the moment, the only network who can tackle EE’s 5G is Three’s superfast 5G service. Three offers the fastest 5G speeds in 154 UK locations and their 5G download speeds are much faster.
Regardless of your network choice, to enjoy 5G speeds you’ll need a 5G ready handset and a tariff.
Half Cooked’s Verdict
EE’s 5G is available in more locations & provides wider coverage per location. O2’s 5G data speeds are faster, but EE has three times more 5G customers than O2 and this could affect the carrier’s average 5G speed in a negative way. We’re giving the win to EE, but it’s a close one.
Voice Call Quality
When measuring voice call quality, rather than our personal experiences we rely on test results published by independent bodies such as Open Signal, OOKLA and umlaut.
According to both Open Signal and umlaut EE takes the top spot again with a score of ‘very good’, while O2 is ranked third with a score of ‘satisfactory’. However, O2 has been improving significantly in this regard. EE was selected as the best in this category, because the network has great audio quality overall & faster call setup times than others. Given how EE is constantly advancing its technology and increasing coverage, it’s safe to assume their voice call quality will only improve further in the future.
Half Cooked’s Verdict
Based on the test results of multiple independent bodies EE is again unrivalled and offers the best voice call quality. It’s not surprising considering EE offers the best network coverage in the UK.
Wi-Fi Calling & 4G Calling
4G Calling (also known as VoLTE) is a mobile service which allows you to make and receive phone calls over a provider’s 4G network. If you have 4G Calling you can make higher definition calls and you get considerably better reception.
Wi-Fi Calling utilises your Wi-Fi network, allowing you to make HD quality voice calls. Basically you can stay connected, even when you have no phone signal but access to a Wi-Fi signal. These two features complement one another, and both EE & O2 offer them to their customers. However, for both networks they are exclusive to pay monthly customers.
As EE doesn’t offer rolling 30 day contracts, you’ll have to commit to a longer term contract on the EE network to take advantage of these features. With O2, both Wi-Fi calling & 4G calling are available on their monthly rolling plans. What makes O2’s Wi-Fi Calling even better is the network’s free Wi-Fi hotspots with their ‘Wifi Extra’ feature. You’ll find thousands of O2 Wi-Fi hotspots, which will ensure you can make calls in even more areas.
Both mobile providers have a long list of supported devices, but if you didn’t get your device directly from their stores there’s a slight chance these schemes might not work.
Half Cooked’s Verdict
Essentially both networks offer high quality 4G Calling & Wi-Fi Calling schemes for all contract customers. However, with O2 you can get them on monthly rolling plans too and the carrier has thousands of free Wi-Fi hotspots.
If you have an eligible EE plan, you may use WiFi on the London Underground provided by Virgin Media. This service is available at over 250 stations, & also extends to O2 customers. Most current phones with an O2 and/or EE SIM will connect automatically.
You might also have access to BT Wi-Fi, if your current tariff supports it. This will let you use thousands of BT Openzone hotspots, but there’s a 3GB data limit per month. If you have an eligible plan, all you need is the BT Wi-Fi app to connect.
O2 has over 16,000 free Wi-Fi hotspots across the country, but you’ll need to register at one of these hotspots first. You can use the network’s Wi-Fi Hotspot finder to find the closest hotspot to you. Once registered, your device will always automatically connect to O2 Wi-Fi hotspots.
“O2 Wi-Fi Extra” is an additional perk that allows your handset to seamlessly connect to hotspots at over 7,000 locations. Wi-Fi Extra will kick in when the service is stronger than the mobile network signal you receive and significantly improve your calls.
If you want to check out other networks with this feature, make sure to check out Half Cooked’s Free Wi-Fi Hotspots in the UK – 2020 guide.
Half Cooked’s Verdict
O2 takes the win here too. EE offers free Wi-Fi hotspots, but to a limited extent. On the other hand O2 offers thousands of free Wi-Fi hotspots and a special service called O2 WiFi Extra.
Tethering or turning your mobile phone into a personal hotspot means sharing your mobile data with other devices such as laptops, smartphones, gaming consoles, tablets etc.
Both EE and O2 will allow you to tether on all their price plans, however on either network you can only tether to a maximum number of 12 devices a month. There’s also a fair usage limit on their unlimited plans. EE’s fair usage is 1000GB (1TB), while O2 will limit you to 650GB/month. As for travelling abroad you’re allowed to use 50GB data per month on EE and 20GB data per month on O2.
Using tethering on more than 12 devices might be considered by EE and O2 to be “commercial use” and these networks may switch your tariff to a business one. The same thing will happen if you regularly pass the fair usage limit.
Half Cooked’s Verdict
Both EE and O2 allow tethering on all their plans with certain limitations. These restrictions are similar, but EE is a bit more lenient. We give the win to EE, but it’s definitely a close one.
Due to EU roaming regulation changes that came into effect in 2017, all mobile networks in the UK must provide inclusive EU roaming. This means be it EE or O2 and whether you’re on a pay monthly or pay as you go plan, you can take your minutes, texts and data with you on your travels.
Both networks offer inclusive roaming in 48 destinations across Europe. EE’s ‘Further Roaming Pass’ adds the USA, Canada, Mexico, Australia and New Zealand to the list. As for O2; it’s ‘Travel Inclusive Zone’ is included in some of it’s more expensive price plans. This feature will increase the number of inclusive roaming countries to 75. You’ll be able to make calls and use data in the USA, Australia, New Zealand and 24 other destinations. However, you’ll have a daily allowance of 120 minutes, 120 texts and data.
The network proposes another add-on called ‘O2 Travel’, which keeps you connected in Turkey and many other destinations for £4.99 a day. This add-on (or Bolt On as O2 refers to it) resets every day at midnight local time.
If you wish to learn more about international roaming in the UK, we have a detailed roaming abroad guide for you to read!
Half Cooked’s Verdict
If you’re on a low-cost plan with EE or O2, you’ll get the basic inclusive roaming you find on any other network in the UK. However, both networks offer more destinations on their premium plans with O2 covering more destinations.
Swappable Benefits & Add-Ons
Swappable Benefits are EE’s special add-ons. If you wish to join EE on an Essential plan you have to pay for each benefit separately. However, EE Smart Plans will let you pick one Swappable Benefit for free. At the moment there are five Swappable Benefits to choose from and you can swap between them once a month.
EE provides five more add-ons, but they don’t come for free. EE also offers 6 months free trials for Apple Music and MTV Play, and the network allows you to cancel them at any time. You can choose your Swappable Benefit and/or switch it via the ‘My EE’ app or by visiting ‘My EE’ online.
Video Data Pass: You can watch as many videos and streams as you want without burning through your data. Works with Netflix, YouTube, Amazon Prime Video, BBC iPlayer, BT Sport, MTV Play amongst others.
Roam Further Pass: You can use your inclusive minutes, texts & data in the USA, Canada, Mexico, Australia and New Zealand, no extra charges will apply.
Amazon Prime Video: You can use Amazon Prime Video on your phone or tablet. If you also have the Video Data Pass, you won’t burn through your regular data.
BT Sport Ultimate: Allows you to watch BT Sport on your phone or your tablet. If you have the Video Data Pass as well you won’t go eat up any data.
BritBox: You can stream the biggest box of British boxsets on your smartphone or tablet and if you have the necessary tech, on your TV too!
EE’s regular add-ons don’t come for free. The network offers a Music Data Pass, a Gamer’s Data Pass, BT Sport HDR Ultimate and BT Sport Large Screen, Apple Music, and MTV Play as add-ons. You can use the latter two without any cost for the first 6 months.
Music Data Pass & Gamer’s Data Pass were ‘Swappable Benefits’, but as of May 2020 the network turned them into regular add-ons.
We think EE’s rewards and extra perks are quite good, but you might want to see what other options are available, we prepared a detailed guide on extra perks such as unlimited social media data usage.
Unlimited Data Plans
EE offers an unlimited data plan with 5G speeds. At the time of writing, there’s only one unlimited data plan offered by EE and it ties you to an 18 month contract. You can also get a ‘Swappable Benefit’ of your choice for free with the unlimited data tariff.
|EE Smart Unlimited Data Plan||Unlimited |
Minutes & Texts
|Unlimited Data with 5G Data Speeds||£35/month 18 Months|
There is a fair usage policy of 1000GB & the carrier will limit tethering to 12 devices per month. Going over either the fair usage policy or tethering limit will put you on the network’s radar. EE says “If you regularly tether 12 or more devices, we will consider this non-personal use and have the right to move you to a more suitable plan.”
EE offers a data rollover feature, but the perk only covers PAYG packs and Flex plans. If you’re on a pay monthly plan you won’t benefit from this scheme. Basically, any leftover data you have from the previous month will be added to next month’s data allowance. You’ll burn through your saved data first. However, if you don’t start your next pack or Flex plan within seven days, your leftover data will expire.
You can read our extensive Data Rollover in the UK guide to discover more about the feature and see which other networks offer it.
When it comes to data rollover we recommend checking out SMARTY and Sky Mobile. SMARTY offers a discount on your next bill for your leftover data and Sky Mobile saves unused data for up to three years!
Free Data Boost
EE’s free data boost feature is exclusive to PAYG packs and Flex Plans. The scheme rewards you with extra 500MB data for every three months you use the same pack (£10 or above) or plan. The boost stays with you if you continue purchasing the same pack and every three months you’ll get another 500MB data as a loyalty reward. For instance, if your data allowance is 10GB, after three months it will grow to 10.5GB and in six month you’ll have 11GB and so on.
EE’s free data boost feature doesn’t include pay monthly plans.
Share EE, Share £20
There’s a £10 Amazon.co.uk Gift Card offer from EE when you refer friends to a £10 or above PAYG pack or a Flex Plan. If you are an existing EE PAYG or Flex customer with a £10 or above tariff; you are eligible for this reward, however, pay monthly plans aren’t included.
In order to invite a friend/family member you must sign up for the scheme first and you’ll receive a unique referral link to share with them. Once your friend joins EE via this link by buying a 30-day PAYG pack you both get a £10 Amazon.co.uk Gift Card within the first seven days.
Family Account – Data Gifting
EE offers a decent family plan option. For each additional plan added to your account you’ll get a 10% discount and also an extra 1GB data. The main account can control all the other additional plans and gift any spare data it has. You can also move the extra data between other accounts too.
You’ll need two or more eligible plans under the same bill payer to activate EE’s Family Account. You can do this by using the ‘My EE’ App or on EE’s website.
We think O2’s greatest feature is the Priority App. With O2 Priority, customers get exclusive offers with popular high street brands, as well as tickets to gigs & events across the country. O2 customers also benefit from many online entertainment discounts. In order to benefit from this scheme you must first download the free app from the store. O2 Priority is available on any compatible smartphone.
With O2’s Refresh feature customers can change their phones whenever they want. O2 Refresh splits a customers bill into two parts – a phone plan & an airtime plan. The phone plan is the cost of the phone you get from O2, and the airtime plan is your minutes, texts and data allowance each month. This way, O2 offers you flexibility by not locking you into a long term phone contract. Your airtime plan will continue, but you can purchase a new device on a new contract if you want to upgrade early.
O2 Bolt Ons
O2 offers multiple add-ons called ‘Bolt Ons’, which you can purchase from the carrier’s store. These are useful when you’re running low on data, minutes or texts. There are also other types of Bolt Ons for calling abroad and for international roaming perks.
Unlimited Data Plans
O2’s unlimited data plans start from £33/month and they are only available to pay monthly customers. The cheapest unlimited data deal locks you in for 18 months. The other unlimited data contract offer is for 12 months, priced at £35/month. Both deals include the ‘O2 Travel Inclusive Zone’ bolt on and 6 months Disney+. There’s no monthly rolling unlimited tariff for O2 customers.
While O2 calls these plans ‘limitless’ you should know that you are allowed to use a maximum of 650GB/month. The carrier also limits tethering to a maximum of 12 devices. Make sure to check out Half Cooked’s Unlimited Data Plans in the UK 2020 guide too!
O2 puts you in control and allows you to design a suitable plan specifically for your needs and wants. You can choose how much to pay upfront, the amount of data allowance you want and you may even spread the balance starting from 3 months to 36 months. You can even change your plan on a monthly basis, no questions asked.
O2 offers a data rollover feature, but only for PAYG customers. Your leftover data can roll over month to month and there’s no base limit to how much data you can roll. As long as you’re on the same bundle, you’ll keep your rolled over data. However, if you change it the left over data will reset. You should also know that unless you renew your bundle on the same day, it will expire.
O2’s family plan offers up to a 40% discount and kicks in when you buy additional plans under the same account. You can add up to 20 plans under one account, plus when you bring in a family member to O2 via your unique referral link, both parties get a £25 Amazon voucher.
Ineligible tariffs are as listed: Business tariffs, standard tariffs, Apple Watch tariffs and any tariff that already has a discount applied.
Half Cooked’s Verdict
While we’re fans of EE’s perks & features, O2 takes the win here mainly because of the Priority App. You can’t really go wrong with either network in regards to extra features, but Priority is the tie-breaker here.
Half Cooked’s Overall Breakdown
EE offers better network coverage, faster data speeds as well as some great features such as data gifting. However, O2 has better extras, provides more flexibility and a wider range of price plans. We are giving the overall win to EE, but depending on your needs O2 might be the better choice for you.
Frequently Asked Questions
Does EE have 5G?
Yes, in fact EE is the first mobile provider to offer 5G coverage in the UK and it has the most locations covered at the moment.
Does EE still use 2G?
Yes, EE still uses 2G for calls and texts. 2G is still important when staying connected in rural locations.
Does EE have 4G Calling?
Yes EE offers 4G Calling to pay monthly customers, but to be on the safe side you should purchase your device directly from EE or else the feature may not work. PAYG and EE Flex plans aren’t supported.
Does EE have Wi-Fi Calling?
Yes, EE has Wi-Fi Calling. It complements the network’s 4G Calling and allows you to connect when there’s no phone signal. You need to be on a pay monthly plan and have a compatible device for it to work though.
Is EE really the fastest 4G mobile network in the UK?
Yes, it’s true that EE offers the fastest 4G data speeds. EE’s 4G download speeds may reach as high as 60Mbps on regular plans and 90Mbps on premium ones. As for 5G data speeds, Three is currently the faster one between the two.
Can I use my EE SIM in the USA?
Yes, EE offers coverage in the USA, but the country is not included in base inclusive roaming. You’ll have to purchase a ‘Further Roaming Pass’ add-on if you wish to use your allowance at no extra cost in the States.
Does EE have an app?
Yes, EE has an app called ‘My EE’. It’s quite handy and it will improve your overall EE experience.
What are EE Swappable Benefits?
Swappable Benefits are EE’s special add-ons. If you have a Smart Plan, you can get one for free and swap between them on a monthly basis. If you’re on an Essential Plan you must pay for them.
Can I add another phone to my EE plan?
Yes, EE lets you add additional SIMs to your account. You can turn your regular account into a family account this way.
Can I give data to others on EE?
Yes, but only to other plans under your account. Also, if you’re running on an unlimited tariff you can gift upto a 100GB/month.
Where can I use my inclusive roaming allowance on EE?
You may use your EE inclusive roaming in Austria, Azores, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Canary Islands, Denmark, Estonia, Finland, France, French Guiana, Germany, Gibraltar, Greece, Guadeloupe, Guernsey, Hungary, Iceland, Ireland, Isle of Man, Italy, Jersey, Latvia, Liechtenstein, Lithuania, Luxembourg, Madeira, Malta, Martinique, Mayotte, Monaco, Netherlands, Norway, Poland, Portugal, Reunion Islands, Romania, San Marino, Saint Martin (French), Saint Barthelemy, Slovakia, Slovenia, Spain, Sweden, Switzerland and Vatican City (Italy). If you have ‘Further Roaming Pass’ all of the above plus Australia, Canada, Mexico, New Zealand and USA.
Does O2 have 5G?
Yes, O2 offers 5G, but at the moment only 100 cities and towns are covered.
Does O2 have Wi-Fi Calling?
Yes, O2 offers Wi-Fi Calling to pay monthly customers. You should also keep in mind that your device must be compatible.
Does O2 have 4G Calling?
Yes, O2 provides a 4G Calling scheme, but only to pay monthly customers. You’ll need a handset which supports the feature.
Is O2 Priority App free?
Yes, O2 Priority is the network’s loyalty programme and it’s free. However, you’ll be using your regular data when you’re on the app.
Why can’t I access O2 Priority?
Most likely you just recently joined O2. It takes some time for Priority’s database to catch up, so your number might not be recognised yet. If it doesn’t work after a few days contact O2’s customer support.
How long does it take for a number to transfer?
It shouldn’t take any longer than 24 hours under normal circumstances. Generally speaking you should be able to make phone calls within a couple hours, texting might take a bit longer.
How fast is O2’s 4G?
According to Open Signal’s October 2020 reports O2 is the slowest major network operator in the UK. The carrier’s average 4G download speed is measured at 18.2Mbps and average upload speed is at 6.2Mbps.
Is it possible to keep your number when switching to O2?
Yes, you can always keep your phone number when switching between mobile networks in the UK. You’ll need a unique PAC code, which will be provided by your old network. You should deliver this code to O2 and they will do the rest.