This has been studied ad-nauseum. Driving while using a cell phone is as dangerous as driving while drunk. It doesn't matter what kind of cell phone it is -- hands-free and hand-held are both as dangerous. The problem is that the drivers mind is distracted by the conversation, not that the hand is in use. Discerning the words from a low-fidelity reproduction of a voice takes a surprising amount of attention. Understanding and responding to the meaning also requires attention. That reduces the attention that the driver pays to whatever is happening outside the car.
A collection of peer-reviewed research papers is available online at:
http://www.psych.utah.edu/AppliedCognitionLab/
An less technical summary is available at:
http://unews.utah.edu/p/?r=062206-1