Are the people you call with your shiny new Nexus 6P saying that they can't hear you? Then you might want to check out several threads on the Google Product Forum and XDA-Developers. A common problem among early adopters seems to be weak and spotty voice quality - that's the voice of the Nexus 6P owner, not the other call party. At the time of writing, several dozen owners across the two sites are reporting very similar problems, both in standard call mode and when using the speakerphone. Google representatives have responded on the official forum, and say that they're looking into it.
Based on some unofficial experimentation by users, it looks like the problem may stem from the phone's noise cancellation setup. Most modern smartphones use multiple microphones in addition to the primary "mouth" mic in order to screen and filter out surrounding noise for a clearer call. Some say that intentionally covering the rear microphone on the Nexus 6P fixes this issue, while others report that it actually causes the problem. Across the various threads there's very little consensus at the moment, perhaps because people are experiencing similar but technically disparate issues. At least one Android Police team member is experiencing similar problems.
One poster on the Google forum, Nicholas Fitchett, says that disabling noise cancellation entirely via a root-level build.prop modification worked for him.
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 its a hardware issue. Google really needs to enable a setting for this or use better software.
New replies are being posted even as I'm writing this story. Hopefully Google can fix the issues with a software update in the not-too-distant future.