Absolutely hysterical!

For those without the acting abilities, one thing that has worked for me for several years (as the Telecom company's telemarketers have ramped up), is to point out (true, actually) that "my company's largest customer is Swisscom... I think it'd be a truly bad idea to switch to <your company name here>". They're off in seconds.

Not funny, but functional. Try it next time.