I can offer no solutions to this. All I will say is - I, personally, am glad we are now living in a time where landlines are not a necessity any more, and mobiles have "do not disturb" features. I have no phone attached to my landline at home, and keep my mobile on "do not disturb" much of the time. If someone genuinely wants to speak with me, they normally leave a message. If not, then I don't call them back.
You could always do what my parents still do - as soon as they pick up the phone and can tell it is a marketing call, they put the phone down, but not on the hook - they leave it off the hook, so who ever wants to talk to themselves can continue to do so. Remember, they are paying for the call. I also suspect that doing this means even if they hang up their side, depending on how the phone system works these days (someone in the know could confirm this for me), it means they can't terminate the call either and move onto calling someone else.
Do this a few times and I guarantee you they will stop calling as it will be wasting not only their time but also their money.
Final option is that TrueCall blocker which was on Dragon's Den a while ago. You can either buy one of these units as a stand-alone entity or some phone manufactures have taken the technology and built it into their phones directly (check out some of the modern cordless BT landline phones that are available).
Edited by deleted (Thu 25-Jun-15 09:03:15)