N.B. Asking this here instead of Movies.SE because I'm wanting to know about the cipher itself and any history, which this site is more suitable for.
In S01E05 of The Wire, at around 45:00, it's revealed that:
the gang that the cops are after encrypt their phone numbers by using a substitution cipher based on the layout of a T9 mobile phone keypad.
In more detail:
Each digit of the number, with the exception of 5 and 0, is replaced with the corresponding digit on the other side of the 5 key ("jump the five''); 5 and 0 are exchanged. Thus, 555-867-5309 becomes 000-243-0751. Devise a one-line statement to encrypt and decrypt numbers encoded in this way.
In the show, this cipher was chosen because it has to be both secure but simple enough for low-level dealers to understand. Is this something that was ever used as a real-life cipher, or anything similar to it, or was it just something devised by the writers?