This is a rather strange error, and we have not been able to discover the cause. Nevertheless and if we have been able to solve it.
1.- Check the volume levels: During a call -> Modify the volume by pressing +. We will check that "Call Volume" appears and increase the sound bar.
It is necessary to do so because this volume control is not available from settings-> sound
If this was not the problem, we continue with step 2.
2.- Turn off and turn on the phone. Basic but effective
3.- Enter settings -> applications -> All tab -> Select Google Dialer -> and delete the storage and cache data