The games take place in the year 2020 and there are special mobile phones equipped with a unique antenna called D-Shot which allows teleportation through "antenna trees". A boy named Shigeki finds one of these phones and is accidentally transported with another boy, Matsukiyo, to another world when he loses a baseball near an "antenna tree".
This world is full of fantasy creatures called Denjū, who all own a D-Shot and use them to call their friends to help them compete in battles. These phone battles are also known as Telefang. Shigeki becomes a T-Fanger and travels the Denjū world, trying to learn more about it. At the same time, he gets wrapped up in several plots involving evil Denjū as well as other humans, who may or may not be T-Fangers.
Credit to andwhyisit for the work on the English translation.