there is a good land line phone called, I think, BT call guardian. can be used on non-BT land lines. just needs caller ID.
it intercepts unknown callers and you can then choose whether to accept or block call.
very simple and gives real peace of mind.
I used to get endless calls from schools, misdialling a software company. I would patiently explain..some were quite insistent, that is the number I dialled. no, you did not, because that is not my number. what's your number they would demand. I'm not giving out any information, I'd say. sometimes, I'd say, hello, this is too-simple-to-dial-correctly. which was a pun on the company name. anyway when I found a recorded message from one of these schools it felt really good to press the block button so I don't get them again, not even able to leave a message.
it's a good system, enables you to take back the control.