Are you getting complaint from those who you are calling on your brand new Nexus 6p saying that they are not able to hear you. Then you might want to go through several threads on the Google Product Forum and XDA-Developers. The usual problem among the early adopters of this device seems to be weak and patchy voice quality - the voice of the Nexus 6P owner, not the person to whom the call is made. Many 6p owners across the two forums are reporting quite similar problems, both in standard call mode and the speakerphone mode. Representatives from Google have responded to this and informed that they would be investigating this issue.
According to some unofficial experimentation by users, it seems the problem might be arising from the phone's noise cancellation setup. Most of the new age smartphones use multiple microphones along with the primary "mouth" mic in order to filter surrounding noise for a clearer call. Some users have reported that by covering the rear microphone on the Nexus 6P this issue can be fixed and others reported that it actually causes the problem. In all of the threads related to this problem, there is no agreement among users. Probably they are facing similar issues, but technically the issues might be different.
One poster on the Google forum, Nicholas Fitchett, says that he found solution by disabling noise cancellation entirely via a root-level build.prop modification. Given below is the statement:
"I turned off noise cancelling via build prop and it fixed it for me. This goes back to the nexus 5 and nexus 6 as well. I doubt it's a hardware issue. Google really needs to enable a setting for this or use better software."
Users are posting about this issue in the Google forum right now. Hopefully Google would be able to fix the issue with a software update soon.