A bug on the Nexus 6P seems to have been fixed by Google, at least that’s what the report says. The bug fix should be incorporated in a future security update release.
Several users of the Huawei Nexus 6P had reported an issue where the voice would echo or the other person wasn’t able to hear the caller. The bug was found in the latest Android 7.1.1 update and seems to be a widespread issue.
Apparently, downgrading to Android 7.0 fixes the issue, therefore it has something to do with the latest update. Google didn’t provide any information, but have closed the issue and updated the Status as Future Release.
In a comment, the Project member stated that the issue has been fixed and will be included in a future release. We are not sure when this update will be available, but you can expect it by the end of this month or early February. The update would most likely also include the latest Android security patch.