In the post on the Google Product Forums, user Allenstevenson91 writes, “After flashing the factory image for 7.1.1. Using speakerphone in calls the audio drops out. Also the caller on the other end hears squealing and noise. Flash back to 7.0 factory images fixes the issue. Maybe not all bugs were squashed for Nexus 6 in 7.1.1.”
He does not appear to be alone as other users who have since chimed in are claiming to be experiencing the same thing. Some have come to the conclusion that it could be a Nexus 6-only problem as other Nexus owners and Pixel owners have yet to experience the problem. It also appears that downgrading to an older build does not fix the problem.
User Tomek Legutko writes, “I’ve tried downgrading the Phone app through ‘Play store uninstall’, unfortunately on 6.0.13 the bug is still here (current version is 7.0.14).” The good news is that Google has acknowledged the problem and are looking into it.