Another way around the credit card problem is to register with talk talk and opt for the mobile service they provide.
You receive a sim card on a pay as you go basis with No minimum contract. It gets topped up with £10 every month automatically by direct debit. The benefit with this system, is you are not tied up into any contract. Also you receive unlimited phone calls to and from your home phone and any other talk talk home phone.
The £10 that is credited every month, can be used for extra phone calls or texts.
This way you are not stuck with a 12 month contract, but still receive the advantages of a contract.