My dad has a Motorola T605 cell phone with Verizon. Most of the time, if he received a call when the phone was on the holder and connected to Bluetooth, it wouldn’t work. Dialing out was no problem. The phone would remain on, but no audio could be heard. If he attempted to re-dial, you couldn’t hear any audio either. So, while voice activation might be working, you couldn’t hear the robot chick on the phone prompting you. Basically, when receiving an incoming call while connected to the hands free Bluetooth, the phone would mute once you answered.
We fixed this by plugging in the car charger into the cell phone. That seemed to fix it.
Some additional details for Google searchers: He recently had Circuit City come on-site install the Bluetooth Hands Free set. He’s got an older Toyota Tacoma, so it doesn’t integrate with his stereo. It merely has the stand, the included speaker the size of a brick, and whatever connections for power. I read on a forum that the yellow wire was mis-documented to be the mute wire, and the orange the correct one. However, since this was wired in the back of his dashboard, and I’m not a mechanic, I figured maybe the phone couldn’t do certain functions when not plugged in. A lot of computers run at half power when on battery. Apparently the phone is the same way.