Sony Ericsson T637

Hey. I got a T637 from a friend in states and I want to use i with my T-mobile sim in London. When i put in the sim it says no network. Tried all the sims but keeps on getting the same message. So it must be locked to At&T network only. My question is that can I unlock it and use it here in UK or can it be only used in states. And for unlocking shall I buy a terminator dongle. Is there any other way of unlocking it .. a bit cheaper way that is.