I would say, from bitter experience, be careful of introducing any call blocker unless you are certain that the PWD cannot "block" random numbers themselves.
My Dad (who doesn't have dementia but is forgetful and anxious) got BT call blocker, which he initially loved. No more calls from all over the world offering to fix his microsoft.
However, he got so keen that he would block any number he didn't immediately recognise, or if he didnt fancy taking the call. I had to get him to reinstate my own number and my sisters when we worried that we couldn't contact him. He was perplexed, cue frustration all round.
It may be possible to get a very simple phone with buttons for specific numbers programmed in but I think it is very hard to anticipate what someone with dementia might do.
