Default Auto mute phone call?

Today, made a phone call, and about 30 seconds in, it went on mute automatically. Thought nothing of it because I have fat cheeks, and could have hit the screen. No big deal, only cut off my credit card number to pay for things.

Then took a call less than 5 minutes later to confirm my order, and it did the same auto mute shenanigans on that call, too.

Anybody else notice this?
Big screen for life. Except when they're broken...