The Phone is a mechanic in Welcome to the Game. It was introduced in the 2.0. update.
The Phone is only used for the Breather, who can call the player to alert them about his desire to kill them. The player is unable to give a response (presumably, this would compromise their safety), though they can choose to accept the call or leave it ringing.
However, leaving it ringing is risky. When the phone rings, it emits noise, which can be problematic if the Kidnapper is lurking around. Luckily, it will first vibrate, which will not alert the Kidnapper, but will allow the player to know when it rings, so they can quickly shut it off. If the phone starts emitting the ringing noise, this will alert the Kidnapper of the player's location, and allow him to kidnap them.
When clicking enter on The Pedo Handbook, this automatically summons the Kidnapper, resulting in an immediate Game Over. Sometimes, however, If the Player clicks enter when the phone is ringing, It will summon not only the Kidnapper, but also the Breather.