I am surprised - they are ringing your phone/the app vs. just ending up as 'missed calls'? I presume you are also marking them as spam to prevent recurrence and that these numbers are not in your contacts? (Not being sarcastic, just trying to help.)
I haven't been marking the numbers as spam, but maybe I'll try to do that to see if the number of calls reduces.
Hopefully marking as spam will help too. One thing I like about Voip.ms vs. GV is that in the former, I can mass block an entire range of number using wildcards, which is rather satisfying especially since I setup the rules to ring as busy vs. just hanging up to keep their systems online a bit longer vs. freeing them to disrupt their next victim.