The system, named Bad Piggy Bank (after its game's porcine villains), is available through the Google Inc. Android version of the game and is limited to customers of Elisa Oyj, a mobile operator based in Finland, American Banker reported. Rovio plans to expand the system's availability to other carriers next year, though it has not made public any plans to extend the system to other game developers. On Dec. 27, "Angry Birds" was the top paid app for Apple Inc.'s iPhone and the top free game on Android, where it is ad-supported. A spin off, "Angry Birds Seasons," ranked as the No. 8 paid app on iPhone and No. 4 on Android. On the iPhone, where Apple limits developers' options for in-app sales, Rovio added the ability to purchase a new "Angry Birds" character, the "Mighty Eagle," for 99 cents through Apple's iTunes store as of Dec. 23. This new character is meant to help game-players get through the game's more challenging levels.