The solution is accessible law, not better anti-fraud tooling. Introduce a spam / fraud button, using it requires your pin and costs $10, but obliges your telecom provider to record the call (preferably including a few minutes before you hit the button), email you a signed recording and if found to be spam deposit $100 onto your account. Your provider may then hand the fee +10% for himself to whatever network the call in question entered his own network from. The last cooperative network in the chain gets stuck with the fee, forcing them to either reclaim the money from the malicious customer, the next network in the chain (in court) or pony up the money themselves.

Result: All routes to non-cooperating networks get dropped within days to weeks and scam-calling stops being a lucrative business basically instantly.