Deep Sky was a Counter Terrorist Unit emergency phone code that was known by all agents. It was a series of numbers that, when dialed, put the caller through to a secure CTU line. The official Deep Sky number was supposedly decommissioned along with CTU when it was dismantled, but the code was reused by Tony Almeida, Bill Buchanan and Chloe O'Brian prior to and during Day 7 as a means of communicating with one another to convey strategies or to plan action.