If you need to be able to use your phone when driving then you need to make sure that you have a hands free driving kit installed. It is against the law to use a phone when driving unless it is used with a hands free driving kit.
There is a wide range of hands free devices available on the market ranging from a pound upwards in to the hundreds of pounds. Some cars (often newer models) come with hands free kits already installed at the factory and often use the Bluetooth function found on most phones to connect to.
If you just want a basic hands free kit to use in an older car then you will probably find ones that connect to your stereo via an Aux port are the best and often do not cost more than a few pounds.
If however you want a completely wireless option, then you may need to upgrade your car stereo to one that supports Bluetooth.