Keitai Denjuu Telefang, which translates to Mobile Phone Beast Telefang and is usually shortened to just Telefang, was developed by Smilesoft, with the first game launching on November 3, 2000, with two separate versions, Power and Speed. Sometime later, bootleg versions of the game titled Pokémon Diamond and Jade were found, which is how it is best known by English speaking audiences.
Source: N4G PC From Telefang to 2019: How An Obscure Japanese RPG Stays Alive Through An Online Community