Strangely enough everyone I know that has signed up has had much decreased calls. Are you sure the main slider is on position to opt out of everything? The language on that slider can be confusing.
Also it expires after 3 years and one has to renew the opt out. I realised that when I suddenly started getting a lot of spam calls Sept 2023. A colleague also recently had a sudden influx of marketing calls and his had expired too the month before.
Obviously it is not effective from the second of sign up, can take a couple of weeks, and companies you have accounts with will still contact you, like Vodacom and Telkom call me, but never a peep from MTN, because I have no active account with them.