Google's Dialer update may have broken Bluetooth calling, as a number of users are reporting in the comments on the Play Store listing for the app that voice calls won't work while they have a Bluetooth device connected. According to the users who are experiencing the issues the voice calls will still go through, but the audio plays through the phone's earpiece and speakers as it would normally if the phone call was being placed without a connected Bluetooth device instead of though the Bluetooth audio.
While this might not be a problem for some users, those who rely on Bluetooth headphones, headsets or speakers to make voice calls hands free are not able to use this particular function, which is a big problem if they need to be able to use their device hands-free. It is worth noting that not all users are having this problem with their phones, and the problem is also limited to Nexus devices and Google's Pixel and Pixel XL so anyone with a different type of device won't be affected by the problem as they will have a different phone app installed on their device.
The update to the dialer doesn't seem to be affecting Bluetooth audio for media, so users can still connect their Bluetooth devices for listening to music, watching video, or playing games, and hear the audio through their connected device as opposed to through the phone's speakers. Bluetooth also doesn't seem to be affected for things like connecting up fitness trackers or smartwatches, so those who have an Android Wear device or other types of smartwatches or wearables won't have issues and will be able to continue using them as intended. The problem is thought to have started with the most recent update to the dialer application which Google notes fixed other bugs that were present in the app. This update also introduced new features and changes, such as the ability to share voicemail messages with other users. If you haven't received the update to the Dialer app yet and you do have a Nexus phone or one of the Pixel phones, be prepared for voice calls over Bluetooth to be disabled as the bug may end up surfacing.