I think the point is that there is a distinction between calls that are intrusive spam or arguably scams but not already illegal, and calls where the content of the call is itself unambiguously illegal.

Bans on unsolicited calls, or do-not-call lists, can be effective for the former, as it moves the behavior from being legal to being illegal, while for the latter, they are already breaking laws, often in a more severe way, and technical measures or enforcement are more important.

A phone company trying to trick someone into paying more per month for something they don't need is likely to be worried about being fined if caught calling a number on a do-not-call list. A scammer pretending to be the tax office and demanding payment of supposedly unpaid taxes is going to be in trouble if caught regardless of rules about what numbers can be called.