They actually do try to enforce it, but right now it's a losing battle. You can spin up a new voip company and start blasting scam calls from an overseas call center in a weekend, then spin that company down before anyone has any idea who you are.
Customers complain to carriers, who then either get the offending voip provider to block the scamming customer, or just threaten FCC action. But the whole cycle is so slow and you're fighting a never-ending shapeless horde.
I think STIR/SHAKEN will help with this if it ever gets fully implemented and enforced, and voip providers are always trying to come up with better fraud detection to avoid the man-hours associated with assuaging carrier concerns, blocking the customers, etc, etc.
