Why does my iPhone jump from full bars to no service?

[tarabeth7779]

This is the second iPhone in a row to do this, so I think it might be a software problem rather than a hardware problem. Apple sent me a replacement phone two weeks ago to replace my one that the volume button broke on. The replacement phone worked for 12 hours and then starting saying No Service. The phone would have service for a few hours and then lose it for 5 or 6 hours and eventually, it lost service all together. AT&T confirmed it wasn't a network issue or media package issue and gave me a new SIM card just in case and it was none of those things. Apple then had me reset the network settings from the phone, do a hard restart, and restore through iTunes as a new phone. When none of those things worked, they did a repair order and sent me the current replacement phone which started doing the same thing about 12 hours after I got it. With this phone I restored 5 times, tried using the phone without any apps or media on it, uninstalled and reinstalled iTunes on my computer and still no luck. It is not losing service as frequently as the first phone, but it stills drops service for at least 4 or 5 hours a day. It seems to be worse when I use the phone more. Is it really possible that I got two defective phones in the mail in a row from Apple, or do you think it is likely that when I get the third replacement phone, it will do the same thing? This is really driving me crazy. This has been going on for over two weeks now - I want my phone to work!