Unlikely. The person on the phone doesn't have the right to make judgments like that. Furthermore, if they were to allow it, that means that everyone who writes gibberish can have their account compromised by someone who guessed that it's gibberish.

Whenever i need to open an important account (say, with the power utility) I make up answers and record them on paper. Simple. Effective.

Or you could just mumble incoherently.

http://www.zug.com/pranks/visa/index03.html