Iphone 13 volume keeps going off and disconnects from my Tesla

My Iphone ringer keeps turning off when I get calls.  The ringer is all the way up.  And when I go out to my Tesla the phone isn’t connected anymore and won’t operate the car.  The car says I am connected.  

I have rebooted the phone and I’ve done a hard reset. It will start working again for about a day and then stop.  Help.  Is it my battery?