Problems using phone for audio

[.FoMoCo.]

Hi there, first post on ford forums!

I recently bought the ASL auxiliary line conditioner mp3 input kit. Great product and service, thanks!
I am getting a rather strange condition with my phone (HTC Desire HD) when it is plugged into the 3.5mm cable. it seems to be receiving some kind of impulse back through the cable, and the phone is acting like it's getting touch input all over the screen and generally having a bit of a spazz. pull the cable out, and it goes back to normal. sometimes it settles down and behaves itself, but there doesn't seem to be any rhyme or reason to it.
I did a bit of research on Google tonight, and it appears that the desire uses a 4 pole jack (trrs) but normal audio cables are 3 pole (trs). apparently trs stands for tip,ring,shield, referring to the contacts on the jack itself. on the phone, my model is :
1 - tip - left audio
2 - ring 1 (closest to tip) - right audio
3 - ring 2 data of some kind
4 - shield is ground
I'm guessing they use the data ring for answering the phone with the supplied headset and some other functions?
The quality of the audio itself is awesome, that isn't in question, just would like to get to the bottom of this weird behaviour.
I've also seen that you can get a trs to trrs adapter? any help with this issue would be much appreciated.
Thanks alot