Keitai Denjuu Telefang

The baseball-loving kid Shigeki finds a phone up in a tree, which briefly transports him into the world of the Denjuu. The Denjuu world is a world that is supposed to be off-limits to grade-schoolers. Bewildered, Shigeki tells his classmates about what happened but only his friend Matsukiyo believes him. They decide to test out whether the phone would really take them to the Denjuu world. After they're both transported there they meet with Crypto, the Elder and many other Denjuu, and their adventure begins.

(Source: wiki.telefang)