The voice Captcha (called the "audio challenge") used by Twitter obfuscates the confirmation code so well that humans can't figure it out either. The voice Captcha system is an alternative to the visual Captcha (the "visual challenge") but it is pointless if an average human being is unable to recognize the eight digits and enter them into the provided text box.
Instead of trying to obfuscate a phrase or a number, why don't we just get the user to answer a couple of simple logic questions such as "Who was president of the USA in 1999?" or "What color is a cricket ball?". Sure, someone would develop natural language processing systems to identify words such as president-USA-1999 and color-cricketball, but with carefully selected questions, it would be indeed very difficult.
So, tell me, human... "If A is greater than B, and B is less than C, which is the smallest?"
Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart