When the robo call problem spiked on my landline, I was getting 5-10 per day. Then I stated answering the phone with the phrase "Please leave a message at the tone." and made a beep noise. This got the calls down to 0-2 per day after awhile (also, I'm on the do not call list). Most people could tell it was really me and, if not, I could just wait until they said something to respond. I've heard that some organizations call numbers to see if and when actual people pick up and then sell that information to scammers. Signaling to them that the number goes to a machine seemed to reduce scam calls.

I have a number of customers for my app CallStop who are elderly and use the app to filter calls on landlines. It works by forwarding calls from their landline to the app, and then customers receive calls via VOIP PushKit and filter them via a whitelist.

Selective call forwarding, if you have it, will get rid of a fair number of things. The do not call list as well is still surprisingly effective.

My Spectrum voice service is $10/mo and has a 30-number white list. Everyone else is blocked.