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?