I do it by using a function on the phone. I answer using sync/bluetooth and then "transfer" to the phone under "options". This "options" menu is only active when the phone is connected through bluetooth. I have a motorola through verizon. I discovered this feature by accident.
I have iphone. If you answer by the phone it automatically goes into privacy. If you answer through sync it automatically goes through the truck, but the phone shows 3 options (sync, iphone, speaker) and be switched from one to the other.