There are features available for most phone types / plans, with varying levels of effectiveness.

For old-school landline phones, look for "Smart Call Block" or a similar feature. These direct all unknown numbers to voicemail, with numbers in your contacts or allow list ringing directly. The feature is phone-specific, so you'll need a device which supports this capability. Unknown calls don't ring but are screened. Call block can be enabled or disabled if you're expecting a call from an unknown number.

For smartphones, various options:

- Numerous third-party call-blocking apps, such as NoMoRobo, Spamblocker, Saracroche, Sereniphone, and more.

- OS-level Unknown-caller blocking. iOS offers a full version of this, see: <https://support.apple.com/en-mide/111106>. Android, produced by the world's largest advertising agency, unsurprisingly doesn't have quite a satisfactory setting, though there are some approximations: <https://www.android.com/intl/en_uk/articles/stop-spam-calls-...>.

- Carrier-level call filtering. "Call Filter" on Verizon, "ScamShield" on T-Mobile, "ActiveArmor" on AT&T. Many Mobile Virtual Network Operators (MVNOs) don't offer network-level spam call blocking. (This is companies such as Mint Mobile, Cricket, Boost, H20 Wireleess, Helium Mobile, etc., see: <https://bestmvno.com/mvnos/>.) Carrier-level filtering may work through an app, at the network level directly, or both. App support may be dependent on your specific device and OS.

VOIP services should in theory afford the highest level of capabilities, though this depends on what your provider enables for you. Rather to my surprise, Xfinity/Comcast actually has one of the better network-level spam-blocking systems by description, "Voice Spam Blocker". This is a three-tiered risk-based system with complete rejection, direct-to-voicemail, and CAPTCHA challenge. (Presumably there's also a whitelist option.) See: <https://www.xfinity.com/support/articles/change-spam-blocker...>.

I turned this on a few days ago. Looking good so far (UK)