12 month old iphone no service to provider.

hi, not sure if anyones having the same problem, but my iphone just couldnt connect to my service provider! ivbe called my service provider and they have confirmed that its not the service problem, ive switched sim cards to another phone and that phone can be connected to my service provider but not m iphone.
ive tried switching on and off my phone, taking out my sim card and replacing it, tried resetting network, and i even updated the software. i just cant think of anymore ideas!
it has never had this problem ever. just recently.
anyone? help?
iphone 3GS iPhone OS 3.1.3