Question regarding postal rule

A advertises his car for sale, B sees the ad, calls and makes an offer. A hears B's offer and say's he'll think about it. A then writes to B accepting the offer, expecting B to receive it the next day. A few days pass, A has not heard from B and calls him up. B is surprised to hear from A, and says "He hasn't received A's letter". B then takes this opportunity, to decide that he's no longer interested in buying the car.

Question: a) does the postal rule apply in this situation? Does A have a binding agreement with B and; can B revoke their offer? is instantaneous communication an exception to the postal rule? In other words, does communicating the revocation through phone eliminate the postal rule of acceptance occurring when posted?