It is against the law to drive a car whilst using a mobile phone, unless it is using a hands free device. Many groups argue that using a hands free device is still not always safe and you are better no to use your phone at all, but often this is not possible, and for reps etc they need to be able to have the ability to make and take phone calls when on the road all day.
There are a wide range of hands free devices out there, and if you have a stereo that has an aux port on it, you can get one from the pound shop! If you re intending using your hands free a lot then I would recommend investing in a good quality one. Many cars now come with a hands free option built in to the stereo and it uses your phones Bluetooth to connect. These systems often will link to your phone book allowing you to ring contacts in your phone. The best hands free kits are the ones that are voice controlled allowing you to speak command such as “call work” without having to take your hands off the steering wheel.