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.
Day 7 Edit
When Tony was taken into custody by the FBI with the help of Jack Bauer, Jack interrogated Tony to find out why he was acting against the United States. Tony covertly whispered the code to Jack so the FBI agents did not notice. Jack, remembering that the code was an old CTU number, called it and was put through to Bill Buchanan was revealed that he was working with Tony to bring down a corrupt faction of President Allison Taylor's government. The three were working covertly on their own authority without the support or knowledge of any government agency to help fish out the corrupt agents.