Many people are experiencing this iPhone 4s signal problem. This happens on both iOS 5.0 and 5.0.1, but mainly on 5.0.1. Follow these steps below to try and fix the problem.
Step 1) Go to Settings > Phone Menu > Turn SIM PIN off. It will ask you for the PUK code which you can get online through your AT&T account. After you enter in the PUK code, go back to settings and turn the SIM Pin off.
Step 2) After you turn off the SIM Pin, turn on airplane mode and wait 15 seconds.
Step 3) Reset network settings by going to General > Reset Settings > Reset Network Settings.
Step 4) Upgrading to iOS 5.0.1 may have actually fried your SIM card. Try and get a new micro sim from AT&T by going into the store and asking them to test your SIM card. This has worked for a small percentage of people.
Step 5) After getting a new micro SIM card, download the latest 5.0.1 from iTunes it you have not already. Restore your phone as a new phone using this guide here.
Step 6) Again follow Step 1 to turn off the SIM Pin.
Step 7) Go to Settings > Carrier > Automatic and turn it off. Then select your network manually.
Leave a comment below if you are having trouble!