O2 is a UK-based mobile phone company that offers pay-as-you-go SIM cards. Subscriber Identity Module (SIM) cards store specific registration information about your phone and are used to connect you to a specific service provider's phone network. Once your SIM card has arrived, there are a few steps to complete before you're ready to start calling your friends.
Save old data
Save important information (such as phone numbers) that was stored on your old contract SIM card to your new mobile phone's internal memory. You will be replacing the old contract SIM card with a new pay-as-you-go O2 SIM card, so you will want to ensure the information is transferred.
You will lose connection to your network at some point between 9am and 6pm on the last day of your contract with your old service provider. Once you see an error message stating "SIM card registration failed" (or something to that effect), remove the battery and insert your O2 SIM card into the marked slot.
Repeatedly power your phone on and off during the afternoon hours. This causes the phone to initiate a network search. Once the search is successful, you will see the O2 logo appear on your phone's screen.
If you do not see the logo appear on your phone's screen, write down your old phone number, your new phone number and your Porting Authorization Code (PAC) number, which can be obtained by contacting your old service provider. With this information in hand, visit www.02.co.uk and click on "Support." At the bottom of the page, click on "Contact Us" and provide the customer service representative with the information you wrote down.