From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Mobile System GB (Japanese: ポケモンモバイルシステムＧＢ Pokémon Mobile System GB) is a system in the Japanese version of Pokémon Crystal through which the player can link up with others by connecting to a mobile phone. It was never implemented outside of Japan, likely due to the fact that cell phones were not as advanced or popular in other regions at the time.
A visit to the Pokémon Communication Center in the Japanese version of Pokémon Crystal, here in an English version hacked to restore the facility and the unused English scripts.
In the Japanese version of Crystal, Goldenrod's Pokémon Center is replaced by a larger building called the Pokémon Communication Center, which includes the machine to use this feature. Another feature of the facility is the Pokémon News Machine, which compiles news from the save files of players across Japan. Reading the news would sent out a player's own save file data, allowing other Trainers to read about the player's adventure.
The system had a special mobile event which allowed players to obtain the GS Ball. This item, given by a nurse in the Pokémon Communication Center in Goldenrod City, may be taken to Kurt in Azalea Town to investigate. After a day passes, taking the GS Ball to the shrine in Ilex Forest summons a level 30 Celebi to battle. Many Pokémon fans worldwide have used a GameShark to receive the GS Ball instead, due to the removal of this system in their region.
The Battle Tower near Olivine City also takes use of the Pokémon Mobile System GB in the Japanese version.
The Mobile Stadium (Japanese: モバイルスタジアム Mobile Stadium) was a feature that allowed the player to link and trade data with friends in a special stadium by connecting with the Japanese version of Pokémon Stadium 2.
In May 2002, Nintendo announced that the service would be shut down on December 14 of the same year. As such, all mobile content services as well as newsletters were discontinued. Access to Battle Tower in the Japanese games was also cut off, as it initially runs through the mobile server. However, it is still possible to use the system to link with friends.