"At least I've got the night-guard fooled, thinking I'm someone completely different".
WARNING! This article contains major spoilers and may contain speculation and educated guesses.
Doug's and Rachel's, or DnR, long-time rival company of the now closing Freddy Fazbear's Pizza, has just opened up on promises to deliver entertainment to all that seek it for years to come. However, the restaurant has a dark secret. A security guard (the player character, identified indirectly as male by the Phone Guy on Night 5) has been hired to watch over Doug's and Rachel's during the night shift. For the first four nights, the security guard receives phone calls from a mysterious man, who describes the ins and outs of being a security guard at Doug and Rachel's. They are required to watch five hostile, jumpy animatronics and a marionette-like creature known only as the Thing, kept behind mechanical, self-opening curtains in a poorly-lit back room. The player finds out that an incident involving the puppet-like 'Thing' animatronic had occurred not too far back in the past, which heavily impacted the company's reputation along with many incidents at Freddy's. To the security guard, the cause of this accident is currently unknown.
As the nights progress, Phone Guy goes into more detail about the restaurant's lore. He is often heard talking to others or managing children, and seems distracted. However, whenever the Phone Guy talks too much about the restaurant's tragic past, he abruptly changes topic. It is Night Five when the security guard discovers a disturbing truth about him as they are fighting to survive.
As his fifth and final night shift begins, the guard expects to hear the Phone Guy greet him, as he normally does, in a mildly positive and somewhat calming way. Instead he hears the Phone Guy recording a message for someone named 'Phil', who is revealed by Phone Guy to be DnR's manager. It turns out that the Phone Guy was oblivious that he had called the guard instead by accident and starts to ramble on about stuff in his message that the security guard had been oblivious about... It turns out that the Phone Guy isn't who he says or acts like he is, but rather another antagonist who seems to be working with the owner of Doug's and Rachel's to take down Freddy Fazbear's Pizza.
The Phone Guy = Freddy's Purple Guy Theory
It is currently unknown if the Phone Guy is secretly the Purple Guy, but it is the most logical explanation since the Purple Guy does seem to have a bone to pick with Freddy Fazbear's Pizza, through morbid crimes, and Doug and Rachel's Phone Guy and possibly his manager Phil seem to share these motivations. The Phone Guy often makes it sound like he is the Purple Guy due to the fact he mentions 'the incident' (which can be found in Night One's newspaper clipping article) in where a kid by the name of Adam Abrey was found severely injured inside of Freddy Fazbear's Pizzeria. This could possibly be referring to the Bite of 87' due to the fact that (according to the newspaper clippings strung up in your office) the incident had occurred in November, 1987. At the end of the phone call, the Phone Guy mentions Freddy Fazbear's once more, saying that they were planning on another incident to aid them in the scheme to bring down the Freddy Fazbear Pizzeria chain. Since it is known to the official FNAF lore that the Bite of '87 was the first major incident that had happened to the company, that would mean that the Missing Children Incident (the second major incident that heavily impacted Freddy Fazbear's) would be the scheme the Phone Guy would be talking about. However, this is all just speculation as there is no constructive evidence of the two being one and the same. Additionally, conflicting evidence in the now-released Five Nights at Freddy's 4 and Five Nights at Freddy's: Sister Location could possibly imply the bite of '87 at Freddy's having been unrelated to Doug's.