It's not at all, for a number of reasons:
* If you're talking to someone in the car, they will be able to see if there's a hazardous situation, e.g. a roundabout or someone pulls out in front of you, and pause the conversation.
* It's always difficult to hear people consistently on a mobile...
As shown on a documentary the other day, a hands free kit is no safer, so you'd better stop that too. And I hope they make it illegal soon.
Here it is: http://www.bbc.co.uk/iplayer/episode/b04c01vs/panorama-drivers-who-kill
and it is largely the conversation that is the issue, i.e...