I know on Apple iPhones you can block all the numbers coming in one at a time, if their "Unknown", then they can't be blocked. Blocked numbers can still leave a voice mail, they just wouldn't know that their call was "blocked". When a VM is left from a blocked caller. It's placed in a folder labeled "Blocked Callers" but will still take up your allotted VM space. So be sure to listen and/or delete.
I am having an unknown caller right now, of course they don't leave a VM so I don't know who it is. I am testing out a way to see if I can block those by making a contact labeled "Unknown" and see what happens. Currently I made it a different ring tone from other callers. So I am hoping that it will work, if it does. I'll make that contact Blocked. Hopefully that will work and I don't have to be bothered by them. This of course could block other legit people with an Unknown number, but I'm willing to take that chance. As a business, that's up to you. I will update though to see if my process works.
Android can block as well, but I don't know the process.
Years back with Verizon Wireless, you could sign into your account and add some numbers to a blocking list. However, you were limited to the amount of numbers you can put on the list. And they would be taken off of the list after 90 days. But could be added back on after the 90 days if need be. Whether VZW still does it or any other company, I don't know.
