This is a well known problem with the Zumo 660 and 665. Your setup would work perfectly with he Zumo 550. I know this because I have the 550 and 665 and Starcom1 digital. One option you may want to consider is buying BT module for the Starcom. This will allow you to bypass the 660 but you won't have the convenience of the the Zumo/phone interface.
I'm not sure about that. When my phone is paired with the Zumo 660 off the bike, I can make hands free calls with the Zumo 660 perfectly fine. Admittedly I didn't have it navigating anywhere, but when testing on the bike where it fails to work well, I wasn't navigating anywhere as well.
I also have the Starcom Bluetooth dongle and that never work well either.
As much as I love the Starcom, this one area has been a pain.
I think I'm going to have to at least try to wire my phone directly into the starcom at least once and eliminate the Zumo from the equation just to see if it is the problem or not.
BS-in-KS, I'd be very interested to hear your feedback once you test it hardwired.