What causes bluetooth phone calls to sound bad even though streaming music is fine?

[VCOUI]

I installed an aftermarket Bluetooth adapter in my e46 with a Business CD head unit. The bluetooth streaming audio sounded *surprisingly* good using Pandora on my iPhone. However anything else, whether it was Siri or a phone call, sounded both distorted and low volume. If I turned up the volume on the head unit, to hear the caller better, the distortion got worse. I'm thinking / hoping it's just a matter of adjusting the settings but don't know where to start.

Have you ever encountered this before? Any thoughts on what might be the cause? I've tried searching the internet but no such luck.

Any an all input would be greatly appreciated!

Cheers,

-V