Setting up unlocked HTC Inspire on T-mobile network?

[~~~BaCkToFoReVeRbOy~~~]

My boyfriend's phone malfunctioned recently, we can't afford the replacement phone. My mom has offered her AT&T HTC Inspire phone (she got it off of ebay, supposedly unlocked and she used it with straight talk, just got new phone and had no use for it anymore.) to him. We have t-mobile but I know that you can use any at&t phone on t-mobile as long as it is unlocked. We went thru the process of officially unlocking it, now I put his sim card in it and it says "No service". I have already changed the APN settings (searched for settings specific to the HTC Inspire) and that has not done anything. I have reset it back to factory settings, restarted it a ton of times. Still not getting any bars, cannot make calls, can't get on internet w/o wifi, etc. Anyone have any idea what this is?? I seriously need help asap!! Thanks!!