Calls w iphone 7/bluetooth - sound speeds up/slows down randomly

I am using iPhone 7, running iOS 11.4. Capacity is 128 GB, I have about 5 GB free.

Whenever I am using Bluetooth (headset or car speakers) for a phone call, the incoming sound will suddenly SLOW DOWN on my side and when it eventually stops, the other person will tell me the sound SPEEDED UP for them.

This never happens with podcast playback. It only happens with phone calls, while using Bluetooth.

It does not happen when not using Bluetooth.

It is not specific to a Bluetooth device -- I have three (identical) headsets and it happens to all of them. And it also happens with the bluetooth connection through the car speakers.

When it happens, switching to speakerphone or regular phone audio stops it. Then if I go back to Bluetooth, it eventually happens again. It will happen multiple times in a phone call. While this has been going on (for weeks) I have tried restarting the phone, closing all apps, updating iOS...it keeps happening. It will reliably happen during any phone call that lasts longer than a few min.

It's maddening, and it has really interfered with some business calls. I don't want to hold my phone to my face or use speakerphone. I want to use my Bluetooth headsets.

Is anyone else having this issue? TIA for any help.