In "The Office," Michael tries to appear hip by setting "My Humps" by the Black Eyed Peas as his ringtone. The joke fits Michael Scott's habit of borrowing popular culture to seem younger and more fashionable than he really is.
"My Humps" was released by the Black Eyed Peas in 2005 as a single from their album Monkey Business. The song features vocals from Fergie and will.i.am and became one of the group's biggest commercial successes, despite receiving sharply mixed critical reactions.
Michael's ringtone is not consistent throughout the entire series. Earlier episodes use "Mambo No. 5" by Lou Bega, but in Season 3 the show repeatedly uses "My Humps" as a deliberately embarrassing example of Michael's idea of being cool. This is why both songs may appear in discussions of Michael's phone, but "My Humps" is the canonical answer to this question.