Persistent spam calls

I am called daily, sometimes more than once a day by various companies contacting me "On behalf of MTN". Each time they want to sell a contract, which I do not want. This is becoming a menace, I can block the numbers as much as I want but they still call persistently. I've asked to be taken off lists, but they keep calling.

MTN, do you even bother taking these pests to task? I really doubt it.