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With HTC devices, over WiFi connection there is a poor call quality. It happens only on WiFi network. On 3G - 2G network the call quality is excellent.
By default, on recent HTC devices, the wifi connection is not set to best performance. With the default setting the incoming audio quality is awful and it’s impossible to do a normal call between two peers.
Open phone settings. "Wireless & networks" --> "WiFi settings" --> menu button and "Advanced" --> enable "Best WIFI performance".