The game allows two- to four-player co-op multiplayer, and, as a new addition to the iOS platform, voice chat, and another new feature - to crouch and full prone - a first for all iOS first-person shooters. It is the sequel to the popular mode on Call of Duty: World at War, the fifth title in the Call of Duty series. Developed by Ideaworks Game Studio and published by Activision for the iOS and Android platforms, it was launched in select countries on December 1, 2011. Call of Duty: Black Ops – Zombies is a spin-off game of the Call of Duty series' Zombies mode and the sequel to Call of Duty: Zombies.