If you are getting problem on Incoming call notification automatically changes to vibrate mode after nougat update then you can resolve this problem by following below solution.
Many user started getting issue what when user are talking on there phone and get any notification arrives, it vibrates After Nougat Update every notification vibrates and to fix it you need to disable vibration setting from your phone . Its a bug in Nougat update and soon google will fix this but till that period you need to turn off your vibration from you phone, To do this in your Android phone follow instruction mentioned at Google
As of Android 7.1.1 DND mode is activated during a call as a temporary fix. Currently the issue has been marked as future release meaning a fix will come in a future Android update.