Iphone not responding to tmobile sim?

I have an att iphone 4 i've reset and restored it, i'm trying to put my micro tmobile sim in it, and its just not working. I got this phone of craiglist, just as I did my last iphone, I paid to have the phone unlocked just as I did before, just not sure why it isn't working?

Are you sure it's a GSM phone? If it is the CDMA version, then it will only work on CDMA carriers like Verizon or Sprint.

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