I had a VTech phone with this feature. I threw it out because its discontinued proprietary-battery was getting too pricey.
This feature is essentially a self-service whitelist: call number > enter code > you're added. Instead of you maintaining the whitelist, others (humans) add themselves to your whitelist.
You need to distinguish between a robocaller and a telemarketer. A robocaller is a machine; a telemarketer is a human. However...and here's the twist...telemarketers nowadays rely on robocallers to dial and queue their calls. Meaning, you should be okay with this feature to block telemarketers (humans).
I make this distinction because not all telemarketers rely on robocallers. For example, I searched for a local HVAC tech and regrettably dialed the number supplied by Google Marketing. Big mistake. With this feature enabled on my VTech, hundreds of humans allowed themselves to leave messages on my answering machine. I disabled it.
@Mouth
I'm not sure which phones offer this feature or even its official name. It's some type of Call Screening...reverse Turing Test...CAPTCHA?