Michael Cassidy (21287) on 7/13/2015 2:32 AM · Permalink · Report

Nintendo's first wireless online service was the Mobile System GB, accessed via the Mobile Adapter GB. This accessory allowed players to hook up their Game Boy system to their cell phone and play online, submit scores, get downloadable content, etc. This tech spec should be applied to Game Boy Color, Game Boy Advance, and possibly Nintendo 64 (Pokémon Stadium 2 uses it through Pokémon Crystal Version). At least 22 games use it.

More here: bulbapedia.bulbagarden.net/wiki/Mobile_Game_Boy_Adapter