Both iOS and Android have a "do not disturb" mode where, when enabled, only phone numbers in your contact book will ring your phone. I just leave it on all the time.

https://support.apple.com/en-us/HT204321

https://support.google.com/pixelphone/answer/6111295 (see "Option 3: Priority notifications only", and pick Contacts Only.)

There's a Do Not Disturb option on ios, that has a sub-option to still allow calls from people in your contacts list.

It is a feature of Google Voice. On Android you can block individual numbers though, or there are apps to filter calls through a ratings database.