How do I block an "unknown" number?

[LegacyDesignz]

This is my son's phone and he keeps getting calls from "unknown". Is there a way to keep these callers from getting through?

[tikibar1]

I believe there are apps in Google Play that will allow this, but since you've posted this in Basic Phones, I'm assuming that's what you have.  For basic phones, your only option may be Usage Controls:

http://support.verizonwireless.com/clc/faqs/Features%20and%20Optional%20Services/usage_controls.html

[SuzyQ]

There is also an alternate to Verizon's voicemail called You Mail - you can use their "Ditch mail" feature which sends Unknown callers directly to VM with a message to unblock and try again; it doesn't give them an opportunity to leave a VM.

youmail.com