The Phone is a mechanic in Five Nights at Candy's 2.
At any time, the player can also rise up Marylin's smart phone, so that it fills the screen. When they do this, the can access the locaion's camera system, which in turn allows them to use activate the phones around the location.
Similar to Five Nights at Freddy's 3's audio cues, using a phone will cause it to start ringing. This allows the player to lure an animatronic, that is in a room next to the phone, to the room with the ringing phone.
Once the animatronic is there, the player has to turn off the phone in the room or the animatronic will break the phone. After awhile, the phone will repair itself, but as long as its broken, the player will be unable to call animatronics to the corresponding room.
If the phone in a room is broken, they also won't be able to prevent Chester and CAT from entering the ventilation system, as the phones are also used to scare these two animatronics when they are trying to fiddle with and enter the vents.
Once Chester or CAT is in the vent, there is no way to make them go back and they will jumpscare the player shortly afterwards.