Hey man,
I too have the Samsung D807 slider phone, and I have the same problem you're talking about.
My phone connects to the Avic Z1 unit flawlessly, but when I use the unit to make a call, it rings once, then says at the bottom of the avic screen (phone disconnected, or something like that) but my cell phone keeps ringing... once the other person answers, the phone reconnects to the avic and I can start the conversation through the mic and the car speakers. This is fine and dandy, I figured it was a built-in feature so that I could listen to the radio instead of the phone ringing.
But then, just this morning, someone tried to call me, it flashed up that they were calling on the avic screen, then immediately said "phone disconnected" and there was no where on the avic that I could push to answer the call... I had to push the Talk button on my cell phone and then it reconnected to the avic and I could continue the convo through the mic and the speakers.
That's when I realized that maybe this was not supposed to do this.
It's weird because all other functions of the phone work with the avic. I could transfer phone contacts (though it would disconnect between single transfers) and all calling functions work. It just disconnects when the phone is ringing (either calling someone else, or receiving a call). Very weird.
I haven't found ANYTHING on the Samsung D807 in here, other than this thread, so I'm not sure if it's a problem with the phone or if there's a setting in the avic that can be changed.