Dr. Pepper:
Disregard the mean and variance. It is unknown.
There could be any random number of calls per month, and each of them could be any random length.
So the question is only answerable in terms of describing how much the customer will save, on average, on each call. (IOW, it isn’t definable in terms of savings per month.) Sorry for that confusion. (i.e. with 6-second billing [with no minimum per call] vs. 60-second billing, the customer saves an average of 27 billed seconds per call.)