Originally Posted by cordelli
If going the sim card route, your phone needs to be unlocked if it's not already, or you need to buy an unlocked one (like $60 or so for a low end model)
T-Mobile is usually pretty good about giving you the unlock codes if you've been a customer for a few months.
I just emailed customer support or whatever they call it after I had had service for 4 months. (this was with the same phone, the Sony Ericsson T610) A day later or so the unlock code and instructions arrived free of charge. I just told them I was going out of the country and needed to unlock the phone.