Phone Guy's Death
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. Here's what we know: on Night 4, you receive your usual voice mail/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.
Who Killed Him?
The phone call itself revealed very little about who actually killed him. The two most popular choices for this are Freddy and Foxy; however, with the recent discovery of the moan in the call, it's also possible that either Bonnie or Chica were responsible. Many fans debate that Foxy was the one who killed him because there is the sound of someone banging on a door in the background of the call. Foxy is the only animatronic who bangs on the door. However, because the Toreador March plays right before Phone Guy dies, many fans immediately assume that Freddy was the one who killed him. Concerning the moan, it's also plausible that either Bonnie or Chica got to him somehow.
However, a contradiction appears regardless of who you believe actually did it. It's safe to assume that, during his shifts, Phone Guy operates using the same mechanics as the player. In order for the situations within the phone call to occur, two conditions need to be met:
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.
Assuming these conditions, there are a few possible ways that Phone Guy could have died following regular game mechanics. Most fans believe one of the following happened:
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 theKitchen 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. Foxy got into the room and killed the Phone Guy. For this to happen, the Phone guy could have opened the door after Foxy left and Foxy could have quickly ran back in and killed him. However, this situation normally only occurs when Foxy's A.I. level is very high, which is usually only possible on the Custom Night. The power also could have gone out and Foxy could have gotten inside as he was at the door. However, Foxy leaves the door immediately after arriving and wouldn't be there if the power went out a few seconds later. Also, Freddy is the only animatronic that attacks when the power goes out. Finally, Freddy's jingle playing right before the screech makes it seem like Foxy had no real responsibility in killing him, as the two cannot attack at the same time. 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.
If one assumes that the Phone Guy was not operating by in-game mechanics, the most plausible theory is simply that all the animatronics attacked him at the same time. This theory is plausible because you hear vocal cues from all animatronics before Phone Guy dies (Foxy banging on the door, Freddy's song, and Bonnie/Chica's groaning).
Since the only source of evidence surrounding Phone Guy's death is a non-visual phone call, there is really no way to confirm what exactly happened to him before he died. His death will be surrounded in pure speculation until Scott decides to shed more light on it.