[unlocked]

I'd add that if you're in the UK and don't have a swipe card (a) you can pick one up free at supermarkets and maybe newsagents, then call T-Mob to tie the number on the card to your phone; and (b) most newsagents can sell an e-voucher (no swipe card needed) that gives you a code number to call in to T-Mob for credit, just like the older scratch-off cards but without the scratching.

Registering a credit card with T-Mob is an alternative but last time I tried their system wouldn't accept non-UK credit cards; that is, they wouldn't register my US-based Visa, MC, or AmEx. Weird since the US-based cards are readily accepted in shops all over the world. If this has changed maybe somebody will chime in.