I usually act insane (comes naturally, yes, I know). Usually I'll just quickly say I think they're aliens and I'm terrified of them, but if I have a bit more time and feel like a laugh I will act very interested in whatever they're buying, but ask all sorts of incredibly stupid questions about their product (can I eat it? Can the car run on it? Does it repell sharks? Has it been approved by the CIA? Will the church be upset with me for using it? etc etc, with great elaborations on each point). When they're convinced I want to buy their product I will start pretending the line is dead; "Hello? Are you there? Hello?" and of course, thinking they have a sale, they frantically say "YES! YES! I AM HERE! CAN YOU HEAR ME?" and after a while I will say yes, I can hear you, sorry, the phone went dead, damn those terrible phone providers and their poor quality connections, now, where were we? You were going to sell me some shark repellant? Then they say no, they want to clean my carpets or install ducted heating or whatever, and I'll say "But I wanted shark repellant! You know how global warming is going to make the sea level rise! The sharks will be at the door step any day now!" (or I'll make up a story about killer bears or aliens or the mafia being after me or my desire to own a yacht or my chronic fictional health condition, and whatever the story, elaborate infuriatingly). If you want to you can keep them on the phone for almost as long as you like as long as they think you are insane but going to buy something. It's fun seeing how far you can go with the insanity without them catching on. Usually you can go to amazing lengths. I figure if they are both annoying enough to take that type of job and stupid enough to fall for it, they are there for my entertainment.
There's really no limit to what you can do. Just act insane and extremely keen to buy, and you'll be able to ramble on about whatever topic you like. Act drunk, act sleazy, act like a stoned hippy, it all works.
Have fun