Call of Duty: Advanced Warfare (often associated with the "CODEX" release in PC gaming circles) is a futuristic first-person shooter that introduced advanced "Exo" movement mechanics to the franchise.
The game attempted to connect to Activision servers to check for updates and bans. The crack edited the hosts file (or patched the executable) to redirect these calls to 127.0.0.1 (localhost), effectively silencing the phone home.