Upon calling back, immediately say: "That must have been YOUR phone because I've got a rock solid signal here!".
I've been using that one for years. 99.9% of the time, the other party admits they are in a "weak" signal area.
BTW, most of those years were with Verizon. They aren't immune to dropped calls. In fact, I'm finding that I have better (stronger) AT&T signal in more places than I had with Verizon. I guess I just pick all of the right places to go!
Mark