Feed back, I Don't need That.... During phone calls

Is this through Bluetooth or wired headphones, or through the phone's own earpiece speaker?
 
Stock phone app, unlocked, on Verizon. It happens on Wi-Fi calling, I can try turning that off. but I think it has happened with that off as well
 
Can you describe a little more what it sounds like? Is it like static, or whining/whistling?
 
It lasts 1 to 2 seconds. Sounds like someone talking but it's so garbled and static I can't make out one word. Seems like the person on the other line doesn't hear it. I would say my on the phone uses moderate and this happens maybe once every 3 weeks