SorryGottaGo.com offers a number of auditory means of putting an end to a conversation.
The conversations with telemarketers will work at best in the USA, but the cat and dog noises, static and other sounds may be universal (of course, you may be suffering from them already).
Via Billy’s blog.
My favourite for annoying people trying to get me take out new subscriptions to magazines is “I’m about to emigrate…”. That shuts them up immediately (wish it were true…)
Paul