I'm going to start off with the assumption (no comment necessary) that none of us EVER talks on a cell phone while driving because it is a distraction and it is not a safe thing to do.
All kidding aside, in this business being glued to your phone is an unavoidable fact. You have to be available 24/7.
But there are things you can do to make things a little easier when driving.
Some people like to use those little wireless micro ear attachments, personally I don't like them - I think the sound quality is awful. But they ARE better than the wired headsets.
I think the best choice is having a wireless mic/speaker combo in the dashboard of your car. More and more cars are being produced with that option.
If your car (like mine) doesn't have that there is another way to go. Motorola makes a Bluetooth mic/speakerunit smaller than a deck of cards that you clip to the sun-visor right above your head.
I got one as a gift last month and the thing has worked better than I could have hoped. No worrying about something slipping or falling out of your ear and once it's turned on, there's just one large button that controls all the functions. Nothing to fumble with - one push on, one push off and you never have to take your eyes off the road.
It's like the speaker-phone on your desk, just much smaller.