I'd fight it.  The police have to prove "beyond reasonable doubt" that you were on the phone, I think the fact that they didn't ask to see your phone and you have no record of the call on your bill should be enough to give reasonable doubt.

I'm with Orbital (especially as I ride a motorbike) with the police coming down hard on this but they have to at least make sure you have a phone with you before they book you for it.