It was the phone, but it also was AT&T network. By that I mean the weaker areas. If the signal strength was good enough, it wouldn't have been an issue. The "death grip" may have dropped a bar or 2, but it doesn't really matter on digital. The signal strength bars are actually meaningless on digital. You either get signal or you don't. It was only the old analog networks where signal strength affected call quality. It doesn't to some degree on digital, but only with a very weak signal where you get signal loss. 5 bars or 2 bars shouldn't be a noticeable difference in digital. I had an iPhone 4 (not on AT&T) from the beginning and could only ever drop maybe 1 bar with the "death grip".
Beamed over the interwebz from my iPhone 4S using Siri.