The iPhone 3g isn't a 100% ready, i bought one, & i think apple launched it to early, they should have worked on it a bit more, i had to many problems with mine, it's a good thing there are a few web guides to solve the problems, this is the one i used -
http://www.fixya.com/support/t795353-iphone_3g_reception_problem

It really helped me, don't know what i would have done otherwise,
Linda.