<Chatgoat / Wolfram> Best Guess: Rrrrrring. Operator: I'm sorry, the number you have dialed is imaginary. Please multiply by i and dial again.
(according to what passes for mathematical humor (my favorite sub-genre), drawn from several sources but primarily from P. Renteln and A. Dundes in their paper "Sampling of Mathematical Folk Humor" in Notices of the American Mathematical Society, vol. 52, pp. 24-34, 2005)
2nd-best guess:
Tell me a joke.Rrrrrring. Operator: I'm sorry, the number you have dialed is imaginary. Please multiply by i and dial again.