E2save have the Talk Mobile (run by the Carphone Warehouse) sim card at £7.50 a month, but if you use Quidco, you get £15 back for a sim only, so your first 2 months will be free, then you can cancel it if you want!
I think that's usually to do with stuff like checking your address details, as it may be different to what they can verify, or your credit card's not going through