This could do so, so much more
Three and a half minutes? Pah! With a little programming, these lines could potentially waste hours of the telemarketer's time! Start with "hello" and wait for the marketer to go into his monologue, and keep listening until the line goes silent. When it does, play a message like "Huh, that's interesting - can you tell me more?" At this point, you go into a loop, playing messages that ask for more information, details, pricing, etc; keep them suitably vague so they work for the largest number of callers. Meanwhile, in the background of the call, play some background noises - pots and pans banging, typing, baby crying, etc. Every two or three "keep them talking" questions, interrupt them speaking and play a recording that asks the caller to "Hold on, I have to (check my stove, feed the baby, answer a call on another line, etc.), could you hang on, won't be but a minute!", and then just wait for two or three minutes - and of course, ask them to repeat themselves when you "return". Every time it plays the "interrupted" message, double the time; eventually you'll have them on the line for quite a while.
Given a few recordings, I'm sure you could tweak it to last the longest amount of time. I'm all for it!