My iphone won't connect to cellphone service/networks.

[krislk37]

I have an Iphone 3g, I originally had it unlocked and working with tmobile. However, my phone would not connect to tmobile. It connects to wifi perfectly fine. When I first bought the phone it worked for a few hours and suddenly went into no service mode and pretty much it stays at no service 90% of the time. I tried contacting the guy who claimed this phone was in "perfect" working condition that sold me the phone and the A**hole ignored all my calls/emails/texts. I put my Bf's (who has ATT) sim card in it and same problem. So then I restored my iphone using itunes and reinserted my bf's sim card in the iphone and same problem. Any Ideas on what this could be? I have asked several iphone repair companies about this and nobody wants to tell me anything. they just tell me to bring the phone in and get a diagnosis. Some guy who wanted me to ship the iphone to him for repairs told me it was an antenna issue and another person said this is a problem with the motherboard and is unfixable, however, I very well know you can find motherboards on ebay for around $30. Any help would be much appreciated.