I suspect they're generally working on some pretty razor-thin margins with these things nowadays [1], and chasing someone who didn't twitch at the bait is probably a losing proposition for them.

[1]: Between all the easy competition and the incurred legal risks. Legal risk is a funny thing because you're basically rolling the dice on being able to collect money without getting in trouble. I wouldn't be surprised a full and honest accounting including legal risks yields a negative net expectation for phone spammers, it's just that even so, some people will get lucky and "win".

They do not prevent the network from connecting to you, only prevent your device from triggering during the call, instead it will be silently ignored.