Background: My wife and I have 3G iPhones but don't live in a 3G service area, so our phones normally work on the Edge network. This weekend we drove up to Washington DC and entered the 3G service territory (as indicated by the '3G' displayed on the phone). My wife used her iPhone to make a call but the call was dropped after a few seconds. She called again and again the call was dropped. She then used my iPhone and it worked fine. Once at our relatives house, I tried a series of experiments. Her iPhone wouldn't maintain a connection more than a few seconds, while mine worked fine. If I switched her phone to disable 3G, her phone worked fine again (just not on 3G, of course). Since we were planing to visit an Apple store anyway, I made a Genius appt to have them look at it. He listened to our symptoms and diagnosed it as a bad SIM card and referred us to the AT&T store in the mall to replace it. I was skeptical but suggested we swap SIM cards between my iPhone and my wifes'. Sure enough, the problem moved to my phone, and returned to her phone when we switched them back. So, off we went to the AT&T store. They replaced my wife's SIM card and it's been fine since.