Can't hear phone calls on Ford Sync

I have a 2011 Explorer with Sync 3, and an iPhone 6S running iOS 11.2.6. Until a few weeks ago, making calls over Sync was a breeze; I could hear people and they could hear me with no problem. Suddenly, that's no longer true - call quality and volume is very variable. I often can't hear the other person at all because suddenly the volume is way down, then suddenly they'll be back and I have missed half of what they've said. If I turn the volume up when they get dim, then I get blasted when they come back to normal volume. I occasionally get loud pops, too. People at the other end of the call can hear me just fine. I have tried a master reset of Sync, and turning off and unpairing the bluetooth/then pairing it again, but there has been no improvement. Any ideas?