hello mate welcome to the forum,
i've had a look in the handbook, and it says this....
if you press the "PROG" button till you hear a beep, it says the display will show the last function selected, so perhaps this will be the "phone" mute.
if not, if you press the "track" < or "track" > buttons till you have scanned thu' to "SM VOL" check what level this is set to, the max i beleive is 12.
The "SM VOL" function is the volumn when you get a phone call if you have a car phone kit installed, so check what level it's set to, and try turnnig it up to max.
Don't know for sure if this will work or not, tbh, but i cannot find any mention of
how to actually turn if off completely.