Phone Guy is the one who talks to you during the first two games and a new one is in the third game. Scott Cawthon did the voice of the first two Phone Guys (supposedly the same person) but not in the third game since the voice is different. And by theory Phone Guy was stuffed into a Freddy Fazbear suit, but it could have easily been one of the other animatronics. We don't know much about Phone Guy, such as his name, so for now fans refer to him by his nickname, Phone Guy.
Another rumour is that freddy was phone guy and he was editing his voice and was helping you since he thinks this is a sour of game in a way and he wants to play this game with his prey. That would explain why when the power goes out he waits to find out what your going to do and then jumpscares you at a random time.
Another theory is that the phone guy was stuffed within the animatronic in the five nights at Freddy's teaser. If the player dies in any of the nights of the first game, he will be stuffed in another Faz Bear suit. It is also known that the phone guy was killed in night four, but there is mention of his body found at the old establishment, except the fingers. The animatronic has five fingers in each of his hand with claws, meaning if phone guy is stuffed in him, the fingers will be damaged or cut off.