Countable vs. uncountable and use of "volume" [closed]

Both are acceptable and the general rule is as you point out in the header. Countables (or mathematically, discrete quantities) get number / fewer and "Uncountables" (or mathematically, continuous quantities) get volume / less.

But

Things get funny with high numbers. People notice the difference between 5 fingers and 4 fingers, but it might take a long time to notice the difference between a thousand people and 999 people. Therefore when using a quantity (calls in this case) that can often be large the difference is taste rather than grammar.

Please note that in volume vs number, there but technically correct. I'm using the assumed rules where people care that there is a rule, but I'm aware that it's not fixed.