Theories

Phone Guy
One of the most hot-topic debates in the fandom is the mysterious death of Phone Guy. The lack of information surrounding his death has made room for intense speculation. On Night 4, you receive your usual recording from Phone Guy. He states that he "won't be around to send you more messages" after this night. In the background, a banging sound can be heard, as if someone or something was banging on the door. As the phone call comes to an end, the Toreador March jingle can be heard, followed by a strange moan, and then the call cuts off into static.

*It's obvious that Phone Guy was killed by an animatronic because of the screech that sounded as the call ended.
 * It's safe to assume that Freddy was involved in killing him in some way. Freddy is the only animatronic who plays the Toreador March. It's not safe to assume, however, that Freddy was the animatronic who captured and killed Phone Guy.
 * Near the end of the call, right before Phone Guy dies, a moan can be heard. This moan is actually one of the noises that Bonnie and Chica make whenever they are in The Office. It's safe to assume that they were involved in Phone Guy's death.
 * Phone Guy needed to have the left door closed. The banging sound implies that Foxy was at the door.
 * Freddy needed to be involved in some way, and he needed to be in a situation where he would play his jingle. The only time Freddy plays his jingle is when he's in the kitchen or when the power goes out.
 * Chica or Bonnie needed to be inside The Office, which means that one of the doors had to be open. If Foxy was banging on the door, then it would be impossible for Bonnie to get in because Bonnie never appears in the West Hall if Foxy is at the door.


 * Freddy snuck into the room while Phone Guy was looking at the monitor and killed him. While this works easily in theory, there are quite a few flaws with it. First off, Phone Guy noticed Freddy's jingle before getting killed. Freddy never plays his jingle before killing the player unless the power goes out. If one assumes that the power did go out, there's another flaw. There is no sound of the power going out heard on the phone call (a loud hum always sounds when the power goes out). Even then, if the power did go out, the call would have cut off much earlier, before Phone Guy dies. However, the call goes all the way through until the screech. It's possible that Phone Guy recorded his messages using a battery-operated machine. However, battery-operated recording devices do not ring like a telephone when being played back. The only possible way that Freddy could have played his jingle and immediately after killed the Phone Guy without the power going out would be if Freddy was in the Kitchen and immediately teleported to The Office. However, the jingle was loud enough to be heard on the call, so Freddy couldn't have been far away when it started playing.
 * The power went out and Phone Guy was killed by Freddy. This seems to be the most logical, as it's the only situation where Freddy will play his jingle and then immediately kill the player. However, there's no sound of the power going out beforehand and there's still the issue with the phone-like device not cutting the call off.
 * Chica snuck into the room and killed him. It's possible that while Phone Guy had the door shut from Foxy attacking, he was looking at the monitor, at the kitchen. If Freddy was in there at this moment, his jingle would have been playing, which would have been loud enough to be caught on the call. During this time, Chica could have made it inside The Office, where she played her groaning sound. The Phone Guy could have noticed this and immediately dropped his monitor, which would have killed him.