Network Issue(sort of)

So I posted my problem under the "iPhone Bugs" thread, but I'm sure it isn't a bug, so I thought I make another thread to see if I can get some answers or help.

So heres my problem. I have an iPhone 3G and like most who owns one, the 3G is what makes it kind of good right? Faster speed, connection, downloads, etc. Well my issue isn't with the 3G. I have pretty good 3G connection in my area. My issue is actually when I disable 3G and try to connect to EDGE. I know that its a slower network and all, but my iPhone can't seem to connect to it. Ok....you know how if you go into settings>general>network and disable 3G, it would switch to EDGE right? well, mines don't do that. The signal bars, AT&T sign, and 3G goes away and it just says "searching". But most of the time it just says "no service". Even after letting the phone sit for mins, even hours, it is still searching for a signal. I would need to enable 3G in order to get service again. I've tested this method on about 5 other iPhone 3G's and all would switch to EDGE when 3G is disabled. Except mines. I tried every tips/tricks, troubleshooting(restarting iPhone, reseating SIM card, restoring iPhone, toggling airplane mode, etc.) and nothing seems to work. It's really not a big issue with me cause as long as the 3G still works, I'll be fine, but I just want to know what could be the source of this problem. Is it the network or is it my iPhone?