Phone calls over headphone jack? I'm looking at buying a new car.  Many of the cars I see now come "standard" with an audio-in jack.  The option of a Bluetooth connection is usually much more expensive.  My question is:  If I plug my phone into the audio-in jack with a cable to the headphones port, can I use the phone "handsfree" effectively while operating the vehicle?  That is, will the incoming sound come over the speakers, and not echo hopelessly into the input on the phone itself?  Or do I basically have to spring for the Bluetooth, either at purchase time or aftermarket?
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.