I think the bit below was probably inserted so that if the Americans asked if we had a "recall mechanism", we could say yes......
An exchange of letters between civil servants highlights fears that the driver might not have had the change necessary to make the call. Macmillan’s private secretary provided reassurance.
Even if by “some misfortune or miscalculation [the pennies] have been expended and one is penniless”, Tim Bligh wrote, there would always be the option of “dialling 100 and requesting reversal of the charge”