My understand use these call centers use VoIP providers in the country though. The traffic crossing the border is likely just an encrypted stream, indistinguishable from some VPN or other generic service.

The VoIP providers are getting paid (by the scammers), your cell/phone company is getting paid twice (by both you and the VoIP provider). It's not like customers are switching to the other spam-less phone option, or giving up having voice service entirely.

I hate to be cynical about it, but literally all the players in a position to track and shut these calls down via technical means have absolutely no tangible incentives to do so.