Some people who use Nexus 6P find problem with their phone. They reported that their phone audio is often distorted or gone wrong during phone calls. They stated that sometimes, they cannot hear the caller voice from their microphone during calls.
This problem is likely caused by the noise cancellation microphone at the back of the phone. At some cases, this problem is just because while the phone is used to make a call, the cancellation microphone is covered accidentally.
The accidentally covered microphone at the back of the phone is usually caused by the fingers that press at it. This is commonly happened because the microphone is placed right under the camera bump where the fingers usually placed while holding the phone.
At other cases, this microphone is covered accidentally when it placed tightly within your head and shoulder during a call. Some people do it while they carrying other things with their hands so that they cannot hold the phone with their hands, that resting it between the head and shoulder instead.
There are some options to overcome that microphone problem. One of them is to ensure that the noise cancellation microphone is not covered accidentally by your finger or other things during a call.
The other option is by disabling the noise cancellation microphone completely by changing the persist.audio.fluence.voicecall build.prop setting from true to false. This option is especially for you who have a rooted Nexus 6P.
If your problem is still remains after you try those options, you can try this following option. The last choice is by making a call to Google support to ask some help. You can ask to get the unit replacement of your problematic hardware and then you will asked to send back your problematic unit within one week after you receive the replacement unit.