I parted company with my TV/'Net provider a few years ago under hard terms. i.e They claimed I owed them, I claimed the opposite, AND they had failed to deliver per contract.
That company called me and made the mistake of threatening me, or at least my reputation as honest, via credit organizations.
I laughed and said, "Considering your company's reputation, I think more people would applaud me".
She said, "Oh yeah? I have a company that will follow you to hell!" I presume this is them.
I have heard that Panasonic has a telephone with a model # something like KX-TG411x CB, which is a very reasonably priced DECT 6.0 cordless phone series that has a "caller block" feature that disposes (sent to 'busy') of unwanted callers on the 1st ring.
Perhaps this, or similar devices, would help?