repeat spam calls

Xfinity has some good spam call blockers (Nomorobo, etc.) But I find it a bit perplexing, looking at my logs, that spam callers call REPEATEDLY, three, four ,or even five times within a minute or so, and are blocked each time. My question is ... why do they bother? That is, once you get shot down once, why do you repeatedly come back for more? It would seem that people paying for spam calls are paying for a lot of wasted time. Under what circumstances might a repeat spam call actually get through? Of course, spam callers are getting paid by the call, so if they call multiple times, I guess they can easily rack up their call count.