Now:
1) You give your real number to someone.
2) Somehow your real number goes into a list used by robo-callers.
3) A robo-call arrives on your real number, disturbing your peace.
After:
0) You give Mozilla 3.99 or 4.99 US$/month
1) You give your Mozilla number to someone.
2) Somehow your Mozilla number goes into a list used by robo-callers.
3) A robo-call arrives on your Mozilla number, that promptly relays it to your real number, disturbing your peace.
You cannot change your Mozilla number, so it is basically an "alias" number, where is the advantage?
Stopping paying so that the number becomes invalid?
But then you won't be reachable anymore by the people you gave that number to.