It is a defence to a charge of using a mobile phone while driving that the call is to an emergency service, as long as the caller is acting in response to a genuine emergency and that it was unsafe or impractical to cease the driving in order to make the call.
Most defences, however, are based on a denial that a mobile phone was being used while driving. In support of such a position phone records can be obtained to show that there were no incoming or outgoing calls or texts. Locus inspections and photographs of the vehicle may also cast doubt on what the police say they saw, or what they think they saw, or, indeed what, they could have seen. Robust and forceful cross-examination may reveal inconsistencies and further undermine the prosecution case.