Aren't cabs dispatched by a central dispatcher? Doesn't the dispatcher log time, location and (usually) name?
Certainly if the cab was scheduled to make a pickup then yes, but it is also common to hail down cabs. Yes, they are "required" to check in with the dispatcher and record their fare but if they don't then there would be no record. Often cabbies try to scam the system, both to get unrecorded revenue and also if it is a short fare they don't want to miss out on their sequence in the queue.