You can delete this post as marketing if you like, it isn't, it is just recommendation, sometimes the dividing line between the two is so small as to be indiscernible, and if you live outside the UK there is not much point in reading any further.
I used to be plagued with telemarketers, I never got any computer related ones, but PPI claims, Double Glazing, Loft insulation, Health Insurance, you name it, I got it, sometimes several times a day.
My number is ex-directory and has been for decades, I have been a member of the Telephone Preference Service for at least 5 years and nothing has ever stopped them. Eventually out of desperation we shelled out for one of these devices:https://www.truecall.co.uk/home.aspx
We have had it since about mid November and we haven't had a single marketing call since. Brilliant, it costs money, but it is worth it for the peace it gives you.
Not only that but it can record calls too, so if you ever have any dispute as to what was said in a phone call you now have real evidence to back up your side of the story. I haven't used that yet but I am sure I will one day. (Unfortunately this feature comes with Windows software so it may or may not be possible to use it from linux I don't know yet, I haven't tried, it is only a memory card that plugs into the phone so you can probably just copy the files off it without using the bundled software ).
It's an answerphone too, although the recording quality is not as good as the one built into my phone, still, you can't have everything I suppose.