3GS suddenly "Searching" & "No Service" messages...?

I have an iPhone 3GS which was working fine until about 2 days ago when it suddenly has the "Searching" and "No Service" messages.... the Wi-Fi works as normal, and my other iPhone 4 has perfect service, but I have NO AT&T phone service now... (no calls in or out)?

I tried restarting the phone, and tried restarting holding down both buttons etc. but nothing works to bring back the phone service (and yes, I did pay the phone bill!!).

I never upgraded to the latest iOS because I was afraid it would really slow down the phone (I have iOS 5.1.1), and I know if I do a "restore" it will update the software automatically.

Any suggestions as to what to do....?

Thanks,

Alan.