> Are you legally required to carry a means of communication? If not, how can this possibly be enforced?

Obviously not... If you have no means to communicate you are not required to communicate. I don't know why you'd think otherwise.

> For example, how fast can I drive to get to a telephone if I don't carry one or otherwise cannot use it?

This would obviously depend on circumstances and how safe you're able to drive without causing more incidents.

This is also why we have courts, and judges, and juries. They look at the totality of circumstances and arrive at judgement.