8800 from AT&T advice!!!!!

Hello everyone, first of all Happy New Year!! I'm a new member to the site, and fairly new to the blackberry world.
My brother-in-law gave me his old 8800 because he purchased a new one. It was with AT&T, and I have T-mobile. I had the unit unlock and I can use my sim card with no problem, but I can't receive or send sms, I think there is an enable key for that right? My question is, can I change the operating system of this unit for that of an unlock or one for T-mobile. This one has some sort of IT config and it always asks me for a password and I can't disable that function it has like a red dot next to it..

Any suggestions??

Thanks so much for any help..!!

I would recommend following the instructions if you search "remove IT Policy" on the forums. This should help.