The numbers that call you that are NOT in your Contact list on your phone go silently to your voicemail. They are not blocked. You are not disturbed.

When they leave a message, you see it and you call them back. It's about the same as if you were on the phone at the time they called and did not want to interrupt the call to answer them. You call them back a few minutes later.

Telemarketers seldom leave a message. If they do, you know what it's about and don't call them back. My VM greeting encourages people to leave a VM if they want me to call them back.

I will turn the feature off occasionally when I know I am expecting a call from someone not in my Contacts, and then switch it back on after they call.