Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of any trouble with this feature.
To answer your question, let me first give you a link to the FULL system manual for this system in electronic (PDF) from. That way if you only have the quick guide that often comes with these systems, this will give you more detail.
Now MOST of the information about blocking calls is on page 34, with a bit more on page 33 and 35.
Once you look at that page (or two) you will see that there are a few different way to add numbers to the blocked list, but perhaps the easiest and most straight forward is to enter the phone number directly. Those steps are at the top right of page 34, and I will interpret them below.
1) When the system is not in use (not on a call), pick up any handset and press MENU # ***** 1 7. That should take you directly into the call block programming of the system.
2) Touch the UP or DOWN arrows buttons to display "block a single number", then press the SELECT soft key (the button below the word SELECT).
3) Press the MENU soft key, then UP & DOWN to display "Add", then press the SELECT soft key button.
4) Enter the FULL 10-digit phone number you want to add to the Block List (using the number keys).
5) Touch the SAVE soft key (to save the number) and then the OFF button (to exit programming).
That should do it.
Please give that a try and let me know if you have any more questions.
Thanks, ***** *****