WARNING:THIS THEORY IS INSANE
I will leave you guys this theory,The Phone Guy is Freddy.
The previous guard, who leaves you voice-mails on your phone, is a mysterious fellow who gives somewhat dubious advice, and a bit of lore. He seems to have worked here for a while, considering the animatronic's free-roam mode would set it very recently, if not in the very near future, and his referring to a certain bite in '87. In fact, most of his messages show that he has seen quite a bit in the history of Freddy Fazbear's Pizzeria, yet he also mentions he has seen multiple security guards "leave" the place, which after saying he emphasizes that most "do not die", a point which he emphasizes. But on the 4th night, he leaves a short message, interrupted by knocks on the door, and concerning what he says and the metallic screech at the end, he seems to have been killed by Freddy. Coincidentally, the 4th night is the night Freddy himself enters the fray, armed with the knowledge of the player's tactics. Tactics reinforced by the tips the former guard gave them. On the 5th night, he sends a message of distorted, mangled speech, screaming, and static.
I think the previous guard is Freddy Fazbear. I doubt he was ever a previous guard. Freddy doesn't animate either because he's A) busy transmitting the messages or B) he simply doesn't feel the new guard is competent enough to evade his fellow robots. And he's probably thinking up what he's gonna say next. A phrase commonly said in the game is "It's Me", a phrase attributed to Freddy. Maybe it's Freddy revealing that he was orchestrating this all along, before he slaughters the poor guard. Freddy is trying to run this business into the ground, by giving it an aura of death. Every single time a guard is hired, he dies at the hands of the animatronics during his shift. When he says "I wonder what the inside that head', he's planting the idea that you won't make it. The 5th message is a final message to the player, a horrible sound to hurt the ears and kill the spirit.
Why is he doing all this? Revenge. In the posters and clippings on the wall, it states that five children were lured by an employee and murdered. Their souls possessed the various animatronics, and began to destroy the restaurant's reputation as a family-friendly joint. They're stuffing the non-possessed suits full of corpses, so that even when the killer robots are stored for the week and replaced with a spare set, they still drive away the customers with their rotting stench and cargo. The fifth child possesses the Golden Suit, the hallucinatory one. And the thing about their plan? It's actually working. The restaurant is slowly waning in the public eye, going bankrupt within the year. There's nothing you can do to stop it, because by the 7th night(2nd of the bonus levels.), you're fired for "tampering with the animatronics." Freddy wins, at the end of it all, no matter what.
