Why Cell Phones are so Dangerous for Driving?

We all know that cell phones can lead to driving accidents. But apparently there is still a dispute about exactly why cell phones are unsafe for drivers:
Is it due to manual manipulation of devices in a car (example:dialing a cell phone)?
In this case the invention of hands-free devices or voice activated commands would reduce the accidents. A person using such devices can talk while driving.

Is it due to the distraction of conversing on a cell phone? Is it the act of actively generating responses to a conversation that's distracting (and hence causes accidents)?
This is more serious. Basically it points to a lack of enough attention while doing two tasks (conversing and driving). If this were true, cell phones (and any other device) that needs active engagement should be banned.

Here's a study that might have the answer.