Are you a BT customer? They've recently introduced a free call-blocking service that needs no additional hardware. I opted in to it a month ago and have since had zero marketing/nuisance calls.
I had an email from BT about this which said this service was: "free and easy to set up". I logged into my account, went through the activation process and at the Confirm stage noticed a tiny box in the top right corner of the screen showing "+£4something a month". It seems it's not the service that is free - BT appear to have used ambiguous wording to say that setting it up is free.