If you want to prevent a person from calling inbound to your office, use the Blacklist module.
First navigate to Admin > Blacklist and click Blacklist number.
This will open the window to add a new record. The number/Caller ID is required and should be in the 11 digit format shown below. The description is optional. After filling out these fields, click "Save." This number will now be blocked from calling inbound, you will still be able to call it outbound.
Find the number calling you
If you aren't sure about the number that's been calling and want to double check before you block anything you can use the CDR Reports module to see who has been calling you.
To find this, navigate to Reports > CDR Reports. In the extra options, change the result limit to 1000 and hit search at the bottom.
This will display the last 1000 call results, inbound and outbound. Find the the number you want to blacklist. In the example below, you can see an inbound call from 17167515500. Copy the text between the <> signs.
Additional Blocking Features
Go to the settings tab in the blacklist module.
Block Unknown/Blocked Caller ID: This will prevent inbound calls that show up as non-standard blocked Caller ID.
Destination for Blacklisted calls: The default is a system hang up, but you can select a different destination.
If you change additional settings, you will need to "Submit" at the bottom of the page and "Apply Config" at the top. This step is not required if you are only adding a new record to the Blacklist.
This module will only block calls that match exactly to the data in Number/CallerID.