New smartphones tend to come with plenty of new features and functions. That said, they also sometimes end up coming with a bug here and there. Google's Nexus smartphones are not exempt from this behavior and although the issues for some started well before today, it would appear that more and more users are reporting the same issue which has led to the more mainstream attention. According to the report details on Android's issue tracker page, users of the Nexus 5, the Nexus 6, the Nexus 5X, and the Nexus 6P are having problems with the call audio via their in-car Bluetooth.
This is apparently only happening to the devices listed that are running on Google's latest version of Android, which is Android 6.0 Marshmallow. It's also not happening to every single user, but enough that it should warrant a deeper investigation from Google. It's worth noting that those who have been affected that are on the two new Nexus phones from this year have had the issue since the phone was launched and they received their devices. This also displays that the issue lies somewhere within the Android 6.0 Marshmallow software, more specifically something recent, as users with Nexus 5 and Nexus 6 devices who may be on Lollipop are not reporting the problem.
Those having the problem are receiving less than desirable results when trying to make outgoing calls with their phones when connected to in-car audio through Bluetooth, as the call will fail to come through the car speakers and then revert to prior audio if any was playing. It's also being stated that phones calls actually do connect to the recipient but since the phone is connected to the car through Bluetooth, no audio can be heard on the device either. This seems to be mostly an issue with calls that are being initiated, and not calls that are coming in as most users only have problems with outgoing voice calls. Aside from the main issue listed here, some users are also reporting having problems with random Bluetooth disconnects. There's no word unfortunately when Google will have a fix ready and begin to start pushing it out to devices.