I talk a lot on my Android cell phone and hear the same thing on occasion. Occasionally I'll also get an echo, a few times I've had what seemed like a conference call with two other people who could not hear each other and only one could hear me just fine. It's totally random and doesn't matter if it's an incoming call or if I place the call, nor does it matter if it's local or long distance for me. I will say that it's been a while since anything like that has happened. Are you in a fringe reception area?
I just chalk it up to wireless anomalies.