Google itself suspects that these are spam calls, because it pops up a bright red "SUSPECTED SPAM CALLER" warning when the phone begins ringing. And yet, I have found no option to have the phone automatically decline the call. There's not even an option to silence the ringer for any call not from my contacts; I actually had to disable my ringer for all callers, by default, and then manually, one at a time, set a custom ringtone for each of my contacts.
This is completely absurd. Why is it so hard to configure my phone to only allow my friends to interrupt me at any moment of the day, rather than any rando? Turns out, I might also hate telephones.