Question about no service on iPhone 3G

[gocatsgo08]

I have an iPhone 3G with iOS 4.2.1 (8C148) and 6.15.0 baseband. When I bought the phone used, it had iOS version 4.3.21 and I downgraded in order to install the iPad baseband so that I could both jailbreak and unlock the phone. The phone is now jailbroken, but when I insert my sim card I cannot get service. The sim card is active and works fine with my other AT&T phone, yet it says no service when inserted into my iPhone. Based upon what I can find out now through reading various forums and other sites, I did not need to unlock the phone since I already have an account with AT&T.

I now have two questions which I am sure someone in these forums can answer. First, am I correct that I did not have to unlock the phone since I already had an active sim card with AT&T? And secondly, would unlocking the phone under these circumstances give me the no service problem? What steps can I take in order to correct this problem?

Thanks in advance for any and all help.

[gocatsgo08]

No replies yet? Wow! Surely someone here knows the solution to my problem. I am getting sooo frustrated having to carry two phones around, one for my calls and one for everything else. My iPhone is basically just an iPod now. Someone please help.

[i.Annie]

You're correct, you didn't need to unlock to use AT&T. You possibly broke your GPS by using the iPad baseband. Secondly, having the iPad baseband shouldn't affect an active AT&T SIM card, unless the device you have is not locked to AT&T but to another carrier, for instance a Canadian or China carrier. Just depends on where the phone is from. You should attempt a restore to a custom 4.2.1 or stock 3.1.3 to see if it will alleviate the no service issue.