I bought a really great little device some time ago that answers all my landline calls with a message saying that it's a call blocker, that I don't accept any sort of unsolicited calls, and to press 5 if you have a legitimate reason to call. Since I had it, not one marketing or scam call has come through, at the small expense that some legit callers don't listen to all of the message and get put off. Regular callers soon got the hang of it though.
Looks like the device and company who made it have since disappeared and you can now only get "call blockers" that rely on databases of scam numbers and unconditionally blocking all private and international calls (or not).
It was such a simple and effective idea, I've no idea why it didn't catch on. It would be a great idea for an app for a mobile too but I can't find anything like it.
Since I'm using mobile for virtually everything now, when it finally breaks, I'll probably just not use the landline at all (except for internet).
You can register your number with the
Telephone Preference Service which is supposed to stop marketing calls - but obviously only from those who respect the rules.