Garmin sat nav bluetooth query

[Android Central Question]

Hi
Have just established that my Garmin sat-nav has bluetooth capability (which my car does not have) and am trying out how things work. I have made and received a phone call but wonder if the volume is turned down or off on the sat-nav will a phone call override this and will I still be able to hear whoever is ringing ?
Also I am told that my voice comes over as very distorted.
Any help or advice appreciated.