When turned on, calls are blocked from any 1 to 9 digit number prefix listed in the Group Block list. For example, adding the prefix '888' will block all incoming calls that start with 888. This feature is OFF by default and is accessible from the Advanced button at the bottom of the Settings page.
Related FAQs:
What is Group Block and how do I use it?
What if the number I want blocked isn't 10 digits in length?