I have an HTC One M8 on Verizon and have not had any complaints about my call quality from anybody while using the Bluetooth microphone in my 2013 XLE. I have a friend that has an M8 as well and he has connected to my Bluetooth and used it for work calls, no complaints. It may be something odd on your phone, like a 3rd party app, that is the root cause of the issue or I could see it being something wrong with the hardware in your phone. I would try to disable apps one by one, ugh, just to eliminate one of them causing an issue. I assume you have tried to delete the pairing and readd it to see if the problem is still there also.