Find and click on the phone number the unwanted calls are coming into
If this is for your phone number it will be under Personal Settings
If this is for a company phone number, you will need to log in with an Administrator login and click on the "Phone Numbers" link in the "Site Administrator" section of the menu
Click the "Edit Call Handling Rules" button
Click the "Add a Call Handling Rule" button
Fill out the call handling rule form:
Change "Apply to:" to "Only calls from phone number:" and enter the phone number you want to block
Leave "At:" as "All times"
Change "Default Action" to "Block caller"
Click the "Add Rule" button to finish adding the call blocking rule