Blacklisting a number blocks or prevents that number from dialing your office. There are a couple of ways to do this using your phone. The first way is by dialing 25625 (BLOCK) on with a desk or cordless phone. Follow the prompts to enter the number to be blacklisted. Keep in mind that you will need to put in the full 11 digit number including the country code (ie. 1-555-555-5555). This will block calls across your entire phone system.
To unblock a number, dial 25569 (ALLOW) and follow the prompts to remove the number.