I own the One Plus One and recently it got an OTA update to Android 6.0. Everything worked fine and my battery life seems to have extended as well. After a while I encountered an issue... sometimes the phone just decides not to ring the ringtone when I get a call, so I was missing a lot of phone calls. As one might expect this was very bad as the main function of a phone is calling and messaging so when one of those doesn't work, there`s a problem.
After a lot of troubleshooting I deducted that when I was connecting to a Bluetooth speaker and then disconnecting by switching off Bluetooth and not disconnecting the device before switching off bluetooth it was still remaining as if the speaker was connected, therefore it was not sending the ringtone to the internal speaker but it was trying to send it to the Bluetooth speaker which now was not available anymore.
I do hope that Oneplus and/or Cyanogen do something about this as it is somewhat frustrating. Before the update I use to just switch off the Bluetooth and the phone would recognize that the speaker is no longer available reverting to the default internal speaker.