Phone calls over headphone jack?
I have tried to do this in my own car - here's my experience with it.

From a "hearing the person" perspective, the headphone jack into the car's speaker system works great. What's even better is that the person doesn't (generally) hear themselves talking either (no echo or feedback). This is using an original Moto Droid, so your mileage may vary.

The problem that I have with this kind of setup is having the person hear me. I still, generally, have to hold the phone up to my mouth for the person on the other side to understand what I am saying. I have found it helps a little bit to enable speaker phone, but then the person sometimes gets the echo feedback. It's really a no-win situation.

Most likely, you'll have to get an aftermarket blue tooth headset.