You can’t make it expensive enough to stop scammers, while not too expensive to impede legitimate calls like businesses, family overseas, or tourists calling home.

There is a very high volume of legitimate calls from Africa, and scammers actually have tons of capital to work with.

And this does not even get into the technical difficulty of defining and detecting a “call” “from” Africa. Voice can be tunneled to U.S. infrastructure and then passed to telephony carriers as a “domestic call.”