It depends upon what your definition of the word “all” is.
There is nothing in the sentence “all calls up to 20 minutes cost 99 cents” to indicate that we can add up the minutes for separate calls into a tally, and if that tally is less than 20 minutes the total cost of all those calls is 99 cents.
Therefore, we can logically deduce that in this context, “all” is synonymous with “every”.
Which should be obvious to anybody who is not a complete retard.
Frenchfries are never sold individually. Frenchfries are sold in groups. Calls are billed on a call by call basis.
Therefore, it would be logical to use the dictionary definition of “all” that means: Constituting, being, or representing the total extent or the whole frenchfries you can eat in twenty minutes cost $0.99.
We have a fridge full of soft drinks at work. There is a sign posted which states: "All Drinks .30". So far nobody has left .30 and emptied the fridge of all soft drinks.
But OTOH, if you really did want to sell all the drinks at once, 30 cents for the whole fridgeful, how else would you word the sign? If it were my sign, I’d write “$.30 each” just to be safe.