I've been wrestling with this problem for several years now and there are several solutions, but no one of them is simple...
I presume you are trying to use earphones/headphones instead of in-helmet speakers to reduce noise...
I will relate to the Autocom wired system for reference, but you could probably use Baehr, StarCom, JB Midland, or others...
Starting with my older Autocom wired system, I found that they have an adapter cable that lets you plug your 2.5mm headphone jack into the cables which run between the main unit and the helmet speakers/microphone. Therefore, whatever you have interfacing with the main Autocom switching/mixing unit (cell phone, iPod, GPS, etc.) is audible through the headphones, as well as, or instead of, the helmet speakers...then you can use in-ear headphones such as the Etymotic ER-6i which reduce outside noise by about 30 dB and give amazingly good audio quality....much smaller than Shure...fits under helmet fairly well, especially if you remove ear pocket pads. Use the iJet remote control to adjust your iPod on the fly, no matter where on your bike you choose to keep the iPod - I keep my iJet remote velcroed next to my tire pressure monitor.
If you want to know how to get the Bluetooth p-hone to interface with the Autocom, one solution is to use the Motorola S705
http://www.wirelessgalaxy.com/bluetooth/s705.asp . Just wire the output of the S705 directly to the AutoCom main unit.
After all of this, I discovered that I simply do not want to be bothered/distracted by either the GPS OR my cellular phone while riding...it's just too much for me. So I just listen to music. For that, I wear Etymotic ER-6i in-ear earphones wired directly to my iPod which I keep tucked safely inside my jacket pocket. I control volume and song selection using the iJet remote control, which has buttons large enough to use with all gloves except heavy winter gloves.
Hope this helps.