I have a HTC Desire 20 Pro. My microphone in calls doesn't work some of the time (speaker mic works fine). This happens intermittently. It varies even with the same callers. So it isn't related to the particular caller's network/phone. I suspect that it could be what service my phone is picking up at the time? I use GoSmart for my service which uses TMobile towers. Sometimes the top of my phone says GoSmart, sometimes T-Mobile? In my network settings it says GoSmart.