Now apparently, and correct me if I'm wrong...I usually am 
....but IIRC you won't/can't be prosecuted for driving whilst on the blower on a 999 call.
It is a specific offence to use a hand–held phone or similar device, when driving. Most offences will be dealt with by way of 3 penalty points and a £60 Fixed Penalty Notice but if the matter proceeds to a Court hearing, the fine can be as much as £1,000 or £2,500 if you were driving a bus, coach or any heavy goods vehicle.
The use of a hand–held phone or similar hand–held device while driving is prohibited. 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".
It is a standard 3 points whether you accept a Fixed Penalty or whether you are convicted of the offence at Court. The Fixed Penalty Notice will be issued if you are seen using the phone whilst driving regardless of whether your driving is affected.
The range of fines is up to £1,000, unless you were driving a bus, coach or any heavy goods vehicle, in which case the maximum fine is £2,500.
There is an exemption for calls to 999 (or 112) in genuine emergencies where it is unsafe or impractical to stop.
Source:
http://www.motorlawyers.co.uk/offences/mobile_phone.htmThe police would argue that you could/should stop your vehicle and turn off the engine before using your phone ~ unless it is a hands free device when its use is permitted although many police forces are now coming down on the use of hands free claiming that using them constitutes "driving without due care and attention"
