WiFi calling problem on the Mytouch 4G solved.
I too have been disappointed with the extremely poor quality of WiFi calling on the Mytouch 4G. I even went so far as to buy a new high-end router and set my phone's wireless connection for the highest priority QOS thinking it might resolve the problem. Bluetooth is a requirement for me and unfortunately the new router didn't make much of a difference in call quality. After two firmware upgrades to the phone in the past year I couldn't believe that the problems discussed here hadn't been resolved.
After doing some investigation online I discovered that severe interference between Bluetooth and 2.4 gig Wifi (802.11g) can be a problem on many devices. It seems the Mytouch 4g is especially prone to such issues.
The solution was switching the phone to the 5 gig Wifi band (802.11a). The MT4G specs don't include this capability in most reviews and product sheets. It also doesn't show up on either the T-Mobile or HTC web page, but the phone works great on that band. For the first time I'm able to use the Wifi calling feature with my Bluetooth headset. Call quality has been just fine.