Hi,
I own an iPhone (i am over 18 just to let you know!) but my arrangement with O2 sounds like how you wish to do it.
I originally opened the account with my bank account using Direct Debit, but after a month decided to switch my direct debit to my business account (still my account but under my business's name) as the phone is for working on the move.
I simply put in a call to O2 and changed my account details from one to the other. I had to wait until after the first bill was taken though, that is their policy.
My suggestion is to ask your mother to sign up for it etc, using her account as the initial direct debit, then after a month change the account from which the direct debit is paid changed to yours.
Now i am not saying this is fool proof as they may question why the name is different but they didnt ask me that at all. Once you have setup the contract the customer advisors are very helpful and as long as the bills are paid on time (which is why they use Direct Debit) they are happy.
PS Im sure your mother will want you to stump up the cash for the first month too.... she ownt fall for that old trick
PPS I'm guessing that if you are 17 you have a Visa Debit card.... not an actual Visa Credit card?