John,
    take a close look at your Garmin install manual (should be easy to get
given
all the .pdf traffic lately).
    The way the pma system works is that you wire your own jack off the back
of the unit and plug your cell phone into that.   you operate the cell phone
the way
you normally would but you hear and talk through your headset.   In other
words,
you dial the number and press send and then you are on your headset. there's
an "off-hook"
switch that I don't completely understand yet but I think it just lets the
back seat people
in on the conversation.

Colyn