That is a known problem and seems to be because the Starcom over modulates the cell phone resulting in garbled comms from the bike. Comms to the bike are fine, as you mentioned.
Someone made a R-C filter to cure this problem and was successful. Unfortunately I do not remember where the thread is but I can look later- you can look now of course.
The basic idea was a capacitor and a resistor (the poster used an adjustable potentiometer but it works as a resistor) and attenuated the signal until it was clear. In a perfect world, someone would make such an attenuator and package it in a 3.5 mm stereo extension but I don't know that there is such a device with an acceptable value for our use.
Poke around a bit on the forum and you should be able to find the info- it was quite recent, maybe two months ago as I remember.
There is also a very able guy on the FJR forum who is very well versed with Starcom although I do not know if he is familiar with cell phone comms. or not.
Finally, you can give Starcom a call directly in England- I have spoken with Tony there several time with mixed results. Also, if you purchased the unit from Jeff at Bike Effects, he may be able to help you.
Brian.
When talking to my wife helmet to helmet or via a Midland CB (Bike to Car) we can hold a perfectly clear conversation. But then using Starcom1 to Phone via Starcom Bluetooth Dongle or via Zumo 660 via wire and Bluetooth to Phone....I can hear her clearly, but she can't understand a word I'm saying.
In discussions with Starcom via BikeEffects they couldn't find anything wrong with the unit, they replaced the headsets which I thought wouldn't make a difference since other forms of communication were fine. The only other variables are Bluetooth or the Mic-02 and Stereo leads to the Zumo. In thinking about it and reading other forums the Mic-02 lead may have been miss labeled and could be a Mic-01 which I think has the wrong impedance (I think). Otherwise I was considering ordering the specific Phone lead and removing the Zumo and Bluetooth all together, but further expense isn't really something I'm looking for.
Since I rarely make phone calls on the bike it hasn't been high on my priority list but it is a bother when I'm out and the phone rings.