One day during Fredbear’s Family Diner’s normal operation hours, a company employee dressed in the company’s signature purple uniform, pulls up to the front of the restaurant in a car, murders a child that was crying outside, then gets back in the car and drives off. Freddy/Fredbear, who was busy doing his usual task of giving cake to demanding children inside the restaurant, noticed the child outside and wanted to save him, but his programming prevented him from leaving. A few witnesses saw that the suspect was a Fredbear employee due to his purple uniform, but no one could identify him and he was never found. The spirit of the murdered child wanders the restaurant and takes possession of The Puppet.
(Evidence: The “Take Cake to the Children” minigame. The purple person obviously murdered the crying child. The fact that he drove up in a car makes it obvious that this murder occurred outside the restaurant, perhaps on a curb near the front entrance. The fact that during the minigame we hear a robotic voice calling out “Save him” while Freddy is giving cake to the other children is evidence that it could be his voice, telling himself that he should save that child but being unable to ignore his programmed duties as restaurant mascot. The reason I believe the child’s spirit possessed the Puppet is because the Puppet jumpscares at the end of this minigame, and it makes sense in terms of events that I’ll describe later)
The company tries to keep this incident out of the press as much as possible with some success, but the story still gets around through word-of-mouth. They soon decide that changing the company name might help, so they officially become “Freddy Fazbear’s Pizza” and change the name of their mascot character(s) accordingly. The original owner is replaced with a new one as well. Regardless, the restaurant starts to suffer some sales decreases. Phone Guy may have been hired around this time for some kind of supervisor position and occasional night duties.
(Evidence: On Night 5 of FNAF2, Phone Guy refers to an “original owner” called Fredbear’s Family Diner, so the restaurant must have changed owners at some point. This would be a likely time for that. I feel like Phone Guy was hired around this time, with occasional night duties, because during both games, he has extensive knowledge about the behavior of the animatronics and all the “shady” events that go on at the restaurant from this point on. And because he always seems to take the side of the company in all his phone messages, trying not to let any bad news slip and come up with “scientific” reasons for why the animatronics act the way they do, he’s probably in a higher position than a low-level employee who doesn’t know anything, and is under pressure from the company to make sure everyone below him doesn’t know the truth)