Iphone 3G Jailbreak problems

I've been trying to get this Iphone 3g 4.2.1 to work after my redsnow, cydia, ultrasnow jailbreak/unlock. I put in my tmobile sim card and it says "waiting for activation, this may take some time." Then either says "no service," or when I turn airplane mode off 'searching.' It's frustrating because I have a first generation that works fine with the same card but is about to die any day now. Any help would be much appreciated.