O2, which is part of the Telefonica group, provides pay-as-you-go and contract cell phone plans to UK consumers. If you've recently acquired a pay-as-you-go O2 SIM card and are keen to get calling, there are a number of steps you need to take to get the service up and running. Once you've done so, you're free to add credit to your SIM card and start making calls.
Ensure that your handset is unlocked if you didn't get it from O2. If your phone is locked to another provider, your SIM card will not work. If your phone is locked, you'll need to contact the service provider that supplied it for instructions on how to unlock it. If you're still under contract, your provider may charge you to unlock your phone.
Take a note of the phone number on your SIM card. This will be your new number.
Insert your SIM card into your phone and turn it on. Your phone should connect to the O2 network automatically.
Dial "4444" from your handset.
Choose how much credit you want to buy and enter your credit or debit card details to activate your credit.
Start making calls.
You can also add credit to your SIM card online or at ATMs and buy top-up cards from newsagents and supermarkets.