Sony Ericsson T637 keeps locking up

I have a situation. I purchased the phone that was already unlocked. I inputted my my old cell phones sim card which was T mobile. Shortly after the T637 would just turn off after an half an hour or so. The only way to get it back on was to remove the battery and pull out the sim Card. After that I found a glbal reset selection and I chose it. Now it want recognize the sim at all this is the error. Insert correct SIM card. Will this have to be reflashed because I reset it. Or is there a problem with the Sim or the Phone?? Please help

sounds to me like it wasnt truly unlocked, sounds like they just flashed it to a newer version of firmware. i had the same error come up on my friends t306 when he tried to put a cingular sim into an att phone, and once we unlocked it, it worked flawlessly.