It is an offence to use a hand-held phone, or similar device, when driving.
If the use of your mobile phone results in the standard of your driving to fall below the required standard you may be charged with careless or even dangerous driving. Drivers (to include somebody supervising a provisional driver) also risk prosecution for failure to have proper control if they use hands-free phones when driving.
A hand-held device is something that is or must be held at some point during the course of making or receiving a call or performing any other interactive communication function.
A device is "similar" to a mobile phone if it performs an interactive communication function by transmitting and receiving data. Examples of interactive communication functions are sending and receiving spoken or written messages, sending or receiving still or moving images and providing access to the internet.
Pushing buttons on a mobile while it is in a cradle for example is not an offence so long as you do not hold the phone. Texting whilst driving therefore is an offence if the phone (or other device) has to be held in order to operate it.
Use of devices other than mobile phones such as GPS’s whilst driving is only an offence if the device performs an interactive communication function by sending and receiving data.
There is an exemption for calls to 999 (or 112) in genuine emergencies where it is unsafe or impractical to stop.