Hi, I work for O2 and hopefully I can clear something up for you.
If you take an additional line with O2 (as in tag another contract onto your existing one) then it does not carry out a credit check on you. The system will automatically check your that your billing history is in good health - usually three months clear billing is good enough. This means that, in the three months prior to applying for the contract, you need to have paid all bills on time and in full.
Sometimes it can go back further but three months is usually enough, and this applies to any contract with O2 in your name. It would make no sense to credit check someone again if they are already with us so that's why they do it this way.
You can have up to four contracts in any one name on O2. Just make sure they do it as an "additional line" on your existing account (they will need to log in with your number first) and I think you can only do this in store.
I'm subscribed to this topic now so if you have any further questions I'll be happy to help as best I can
xx
