About a week and a half ago, me and my wife were out at a store and both of phones lost service at the exact same time. She was inside the store and I was in the parking lot. The phones then went into a cycle of 45 seconds of full 3G signal and then 20 seconds of "Searching..." for signal. It was a true total loss of signal, no calls or data during that time.
I've contacted AT&T several times. They have claimed it was tower problems... It has happened over an area of 30 miles. They have claimed it was my SIM card... It has been replaced twice. They have claimed it was my iPhone 4S... It has been replaced. The issue is still happening. Supposedly the case has been elevated to engineering but I have yet to hear anything from them even after attempting to contact them and get some answers.
I have found two temporary workarounds. If I turn off Cellular Data the phones will stay connected. I just won't have any mobile data. If I swap our SIM cards and then swap them back, the phones work correctly for about a day and a half. Then they lock up and require a hard reset and start the cycle again.
I have about had it dealing with AT&T on this issue.
I have a 4 and 4s and the 4s 3G hangs a lot. Apple switched out my phone after having me do a "restore as new". Nothing has fixed it. My wife can surf faster than me on her 4 vs. my new shiny 4s. Lame.