That Time Nintendo Took the Game Boy (and Pokémon) Online kotaku.com

As the late ‘90s gave way to the new millennium, Nintendo decided it was high time its long-standing Game Boy handheld made the jump to an online space of its own.
This move was likely inspired by the record-setting sales of Pokémon, a series introduced in 1996 that had almost single-handedly revitalized the Game Boy brand.
The Pokémon series’ focus on battling and trading with others via the Game Boy link cable made the idea of producing an online component for the Game Boy an obvious next step.
A 50/50 joint venture, Mobile21 was created solely to bring the Game Boy online and produce games compatible with its new mobile network.
Thus the Mobile Adapter GB was born — a device that allowed Japanese players to plug their Game Boy Color (and later Game Boy Advance) into a cellular phone to access a host of online services.