Speaker volume decreases

[Grumpy II]

My wife and I bought moto z2 (I have a force, she has a play) about 2 weeks ago. We use the speaker a lot for business conference calls. 10-12 minutes into a call the volume decreases and there doesn't seem to be anything we can do to get the volume back up. Call back and same thing happens again. This happens about 80% of the time, on both phones. HELP!