My service provider offers <$0.01/min. calls to many countries.

I have a hunch that no telco wants to entirely block a foreign telco because they would lose money and because of the ensuing chaos.

Robocall friendly service providers intentionally mix robocalls with legitimate traffic to avoid terms of service enforcement. e.g. Robocalls are laundered by splitting across n carriers so spam (identified by bad ASR/ACD stats) stays below each carrier's threshold. https://news.ycombinator.com/item?id=12339739