Hubby and I sleep days, so we keep our phones on constant "Do Not Disturb" mode. If a caller isn't on our contacts list, they get sent straight to voice mail without our phones making a peep. Later, we Google the numbers that called us, and quite often discover that they're scammers or telemarketers. I've even had a bogus "collection agency" (actually scammers!) calling me multiple times a day. Scammers and telemarketers get put on our blacklist, which means that if they call again from the same number, they get a recording saying our phone has been disconnected or is no longer in service.
Quite often they'll call again from a different number, but we just keep right on blacklisting the new numbers as the calls come in. It's not perfect, but it does eventually slow down the number of calls.