It is an offence to use a mobile telephone [or similar for example a Blackberry] whilst driving. Conviction will lead to the imposition of 3 penalty points and a fine the level of which is in the discretion of the court.
It is a defence to show that your mobile was being used on a “hands free” basis. It is not a defence to show that the quality of your driving was not impaired by the use of a mobile phone. In limited circumstances it can be a defence to show that you were phoning the emergency services – but you will have to justify why it was impractical for you to stop your vehicle whilst you made this call.