There is a way to "block" harassing phone calls through Comcast Digital Voice. You must know the phone numbers that you want to block ahead of time. Once you have determined the numbers to be blocked, Comcast will automatically relay a message saying you "are not presently accepting calls" for up to 12 numbers total.
Lift your receiver and wait until you hear your dial tone. Press *60.
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Follow the voice prompts to add the first phone number you want blocked. Include the area code and seven-digit number even if you are adding a local phone number.
Listen for confirmation that the number has been successfully added and move on to the next phone number until all the numbers you want blocked are entered.
If you decide that you no longer need this feature on your phone, it can easily be canceled by pressing 80 and pressing number 3 on your phone; this will cancel call screening on all numbers on your list. You can also unblock individual numbers by dialing 60 and following the prompts to delete certain numbers from your call screening list.