A code 47 was a type of high security communication used by Starfleet, they required voiceprint authorization to open the channel and could only be received by officer ranking captain and above, no computer records of code 47s were recorded to maintain their security.
In 2364 captain Walker Keel of the USS Horatio used a code 47 to secretly contact captain Jean-Luc Picard of the USS Enterprise-D to request his presence at a meeting to be held on Dytallix B to discuss several officers’ fears that a conspiracy was active within Starfleet. (TNG episode: "Conspiracy")
In 2370 Picard used a code 47 to contact admiral Alynna Nechayev to talk to her about another conspiracy within Starfleet he and his crew had discovered. (TNG - The Space Between comic: "An Inconvenient Truth")