AirPods sound in super slow motion

Occasionally the sound on my AirPods during a call is in super slow motion, like I and the other caller are using a sound modulation device. It sounds really creepy and makes it impossible to understand one another. What could be causing that? It probably happens about 20% of the time intermittently. Using an iPhone 7 on AT&T, and I don't have the same problem if using the phone without AirPods.