Well, I have it sorted.
I simply do not answer any incoming calls, and have turned the ringer down to near inadibility. I've alerted all my acquantences to my mobile number and use the mobile for outgoing calls, too. I need the landline for internet, so cannot cancel the service. My outgoing calls on the mobile I've worked out to be only 25% of the cost of a landline call, so, for me, it's win, win. And for BT, lose, lose.