You need to enable screening by going to https://voice.google.com/u/0/settings .

I've got screening enabled and still get at least 3 spam calls per day, usually more.

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.)

Yes, it rings my phone. Most are annoyingly early in the morning, and 90% of the time it comes from random numbers in the same area code as my phone number.

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.

One more thing to check: in the Legacy GV webapp (google.com/voice/b/0/redirection/voice) in the Groups tab, the Screening is On for anonymous callers in addition to the original place that I listed above?

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.