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I think a really funny knock-knock joke can actually open more doors than a high GPA/GMAT (kidding).
Some interviews are done by students, some by alums, some by ad-coms. Ad-coms have pretty diverse backgrounds. At the end of the day, they're just people so you have to feel them out to figure out how much humor you want to use. I think in general it's good to not be too super-serious so if being funny is one of your strengths, then definitely inject a little humor where necessary.
Relax and have fun with it and you'll do fine. One other tip for phone interviews, some people find it helpful to put on a suit. Psychologically it can help get you in "interview mode" better than wearing sweats and a t-shirt. Good luck!
Johnny's right -- just follow your interviewer's lead. If s/he is very serious, then humor from you will come out as frivolous. If the interviewer is friendly, then some humor is great.
The other thing about phone interviews -- you've lost all visual cues. So you'll need to be super-careful. I notice that people on the phone tend to ramble since they can't see me nodding my head and letting them know that I've heard enough. In this instance, answer briefly and then say "I could give you another example" or something along those lines. Also, it's extra-important to slow down. Again, the interviewer won't have visual cues to help him/her understand you, so you need to be extra-clear.