In most instances O2 UK SIM cards will start working as soon as you have topped up credit .
If this doesn't work you can activate your SIM card by simply calling 248 from your O2 mobile after you arrive in the UK (this number is printed on your top-up card). Once you have done this you need to buy credit using the top-up card from virtually any convenience store, newsagent, pharmacy, petrol station, post office or pharmacy. You should be able to have all this organised within 5–10 minutes of arriving at the airport.
If you are on the International SIM plan we recommend that you add £15 as your first top-up as this will give you 30 days of free calls to other O2 mobile phones in the UK.
You don’t need a credit card to activate the O2 UK SIM card and you do not need to register online.