I use a bit of kit called TrueCall. It's available as an add on unit and also is built into some of the BT phone offerings.
It allows you to have reasonably graduated control over who actually gets to ring the phone and disturb you.
It is a cost for the kit plus an ongoing subscription. Only you know if it is worth the cost.
Mine is set to allow all on my address list, reject any withheld, and everyone else gets an automated message to state your business before ringing the phone.
That seems to weed out most of them, persistent ones that jump the hoops are then put on a barred list at the press of a button.