I've been having the exact same problem for over 1-2 months now. About once a day or two, on a phone call, I hear the person at the other end slow way down -- to the point of being unintelligible -- for about 5-10 seconds. After another 3-5 seconds of silence, the audio returns to normal. If I'm talking during this time, apparently I sound sped-up (like a chipmunk) to the person on the other end.
This is really causing issues when I have business calls or conference calls. It's hard to be confident in the AirPods when this kind of thing happens randomly.
It happens both during normal phone calls and also during calls using e.g. Skype. It does not seem to happen when listening to music.
I didn't have this problem for the first 6+ months of owning these AirPods. Was there some kind of software update 2+ months ago that caused this issue to appear?