Distorted/scratchy sounds on phone calls (Bluetooth)

Ever since about a week ago, my Bluetooth speakerphone calls started sounding highly distorted in my 2006 Prius. The voice quality is terrible, kind of scratchy or like static. The person on the other end of the phone can here me clearly, though.
The problem is only experienced when using the Bluetooth speakerphone. The radio, CD player, and AUX input all sound great. So based on this, I don't think the problem is with the speakers. Also, I trued using my phone with another Bluetooth headset and it sounded good, so I don't think the problem is with my phone either. It seems likely that something in the Prius audio circuitry related to Bluetooth speakerphone has gone bad. This is only a guess, but it sounds like maybe an amplifier has blown or something like that. And as I said, this problem only started happening recently. Prior to that, the Bluetooth sound was good.

Anyone have any ideas or share this experience?