I get quite a few of these calls, even though I'm signed up with that BT scheme whereby you can opt-out of sales calls and the like.
I'm not too concerned, mind, as they can be lots of fun. If I don't have the time to deal with them, or if I'm not in the mood, I simply tell them - in no uncertain terms - to, erm, "go away", though as you can imagine that's not the real way I put it to them.
If I've got a bit of time to spare, I sit back and play them like a hooked fish. My record, so far, is keeping one of them on-line for 14 minutes. It's often very difficult to keep from laughing when they get very upset because you're "not listening" to their instructions properly. I've had them actually SHOUTING at me when they realise that I haven't been looking at/selecting/accessing whatever they've been telling me to do. Naturally, when I feel they've had enough I usually tell them what I think of them until they hang up in disgust. Great fun!
And why are they always called Donald, Simon, John, Mary, Samuel, Diane, Frederick, and Alexander? Heh!