Most (reputable) mobile phone retailers will ask you to provide at least 2 forms of ID, one for your address and one of your signature. It's fair to say that they'll accept your debit card as proof of signature, but the only proof of address they'll accept without finding out how old you are is a bank statement from another bank (ie. not the same bank as your debit card, so you'd need to open another account). Although it is fraud to take out a direct debit by deception, whilst you are under 18 the company technically has no legal claim to your money.