I bought a box off Amazon for $25. It goes between the phone line and the phone. I can whitelist and blacklist numbers. When a number that isn't on the whitelist calls, they get a message saying to remove me from their telemarketer list or if they're someone I know to press '2' to put their call through. I can then whitelist anyone who asked to bypass. After 2 weeks, pretty much everyone I knew was whitelisted.
Kills robocallers dead, as they don't have ears and can't press 2.
Kills most telemarketers who are live calling. I've had two try to ring through. I just press the blacklist button, it hangs up for them and if they call again they get hung up on immediately. They get one ring and then click -dialtone-.
Other than those 2, my phone hasn't rung once from someone I don't know. Only the guy repairing my vacuum cleaner got confused about 'press 2', but he's not the brightest bulb in the factory.