Some rare Denjuu, known as Secret Denjuu, can only be recruited if their phone numbers are found by the player first. These numbers were never given ingame, instead they were released gradually on television commercials, in issues of Comic BomBom, packaged in toys, etc. The number can then be manually dialed at any time, and the Denjuu will be added to the Contacts list.
Phone numbers can be manually deleted from the Contacts list, to make more room or if the player simply wishes not to use that Denjuu anymore. The last saved phone number can also be lost if the player loses a battle. If they lose the number of their only Denjuu in this manner, it results in a game over.