I suspect you need a transformer for the audio from the phone to the
audio panel... The impedences are quite different between the speaker
and mic on the cell phone and on aircraft audio panels... How about
simply adding a Lightspeed headset with the cellphone connector already
integrated into the electronics... That's what I use on Fat Albert and
it works slicker n you know what... Give JA AIR, or Air West a jingle
and talk to their tech...
OR, Jump over to the homebuilt forum here - R.A.H. - and give Jim at
RST a buzz, he probably has an audio box for (ahem) non certifed
aircraft that can be plugged into the com3 jack without being
"connected" to your airframe...

denny