From Wikifang, a definitive guide to Telefang, Dino Device and Bugsite
Denjuu (Japanese: 電獣, Romaji: denjuu, Bootleg: Electric monsters/E-Monsters) are monsters found in the Denjuu world. They come in many shapes, sizes, kinds and levels. They are able to communicate with human beings, and the player can be their friends by getting their phone numbers.
 Phone Numbers
Each Denjuu has a telephone number which the player might receive winning after a battle with it, or through other means. Every Denjuu has a different phone number.
 Calling them
When a player enters a battle, the current Denjuu partner will automatically enter the battle, together with either 0, 1, or 2 other Denjuu friends of the player's, using his/her D-Shot to call them. How far away they live from the current position of the player would determine the number of turns before the Denjuu reach the player and join the battle. They may also get lost along the way and might turn up late, or not reach at all.
The player can call a Denjuu friend outside of battle to get it to be the player's current partner. It will follow them from the behind. It's automatically entered into a battle.
Only one partner can be with the player at any one time.