I just had same problem. Phone is only 6 days old, running latest, newest firmware. Funny is that nothing helped, no restart etc. and later it disappeared as it started. And all was normal again. Same story was day before. Could not understand what is the cause or when/why it starts. Only common thing was that I was driving and I did not use any so called hands free sets. Now I made some changes under Security>Permission>Safe>Call Monitoring so, that all apps require to prompt for permission to control calls/volumes etc. Will check, does it help, can it be the reason, as there are on the list some GPS navi apps what can try to control my calling habits for safety reasons.