First, with a plunger mute. The adults in the Peanuts cartoons were "voiced" by a trombone with a plunger mute, which gives the impression of a voice without necessarily sounding like it's saying anything in particular. (http://en.wikipedia.org/wiki/Peanuts#Television_and_film_pro...)
Second, it's actually possible to make it sound slightly like specific words by altering the shape and size of your vocal cavity while playing a note. When this is combined with slight glissandos, it can be made to sound a lot like speech. In fact, there is a piece for solo trombone called "General Speech" which uses this technique to recreate General MacArthur's farewell speech at West Point on the trombone. You can read about it and hear it at http://artofthestates.org/cgi-bin/piece.pl?pid=11
To be fair, neither of these techniques produces anything you'd be able to understand in a conversation, but the point is that a sufficiently skilled trombonist could definitely say something that could be recognized as "I Owe You" on the instrument.