I was told a while back to answer the call and after about 15 seconds say "there is someone at the door, let me get rid of them and I'll be right back". Then set the phone down and just walk away while they are just setting there waiting for you to come back.

I did this once and after about 2 min I came back and the guy was still on the line. I told him that I was having trouble getting rid of a pushy sales person and that I would be right back "again". After another couple of min I came back to the phone and he had hung up. I guess he got the message.

Oh yea, while they are waiting for you to come back, you could also just start talking loud about chickensh!t salesman that keep calling you.