On Android 7.1.1, the Nexus 6P and 5X have an in-car Bluetooth issue unresolved since October
For most users, the Nexus 6P and Nexus 5X either won't connect to in-car Bluetooth systems, or they connect, but phone calls can not be made via the car's audio system. This seems to happen with most car models, regardless of maker.
As mentioned in the lengthy discussions found at the Google Product Forums and the Android Issue Tracker website (see the source links below), the issue has been around since late October, when Google released the first 7.1.1 Nougat developer preview. Users were hoping that the problem would be fixed with the arrival of the January security update, but that wasn't the case.
Google may be planning to finally fix this Bluetooth bug with the launch of Android 7.1.2. Unfortunately, there is no word on when this new version of Nougat is expected to arrive, though we hope it will happen in the near future.
sources: Android Issue Tracker, Google Product Forums