I'm assuming this is being sold here, and that's what the OP is about (?).
I almost bought a separate call blocker unit one time and found that I didn't do proper research because it would not have worked for me. But I do have phones with a 'block' button, so that is good, as every one has it.
Anyway, the thing people need to make sure they know that the call blocker unit has to be hooked up to their MAIN phone unit. In my case, that is upstairs in my old office, hooked up to my modem and router. For that reason it would have been profoundly inconvenient to use, as I'm downstairs near one of the other handsets 99.9% of the time now, and I would have had to run to the back of the house (not a small house), and then up the stairs and to the other side of a huge room. By then it would have been too late. Plus, due to injuries I can't actually 'run' anyway and it takes me a minute or so to get up the stairs.
Just make sure you know if this would work for you or if it would be more practical to just get phone handsets, as mentioned by others, that have a block button right on them. 