An acronym used by the military, specifically the U.S. Navy, meaning Internal Communications System. This applies to aircraft, not ships. Ships use something called sound powered phones, (as well as the typical bitch box and conventional telephone systems,) which aren't really sound powered at all since they're coupled to amplifiers these days.
(Note: New people are often sent looking for sound powered phone batteries after first getting to a ship. This can also lead to being sent looking for other silly things, such as radar/sonar contacts, buckets of steam, keys to the ship, etc. This is mainly used to make fun of the little kiddies who have no social lives as of yet, because they just got out of boot camp. Strange parallel.)