The phone guy says on night 1 that there was someone before Justin Smith (main character) and finished his week, but complained that the animatronics moved. Jollibee Foods Co. tested them and told the results to him and Justin.

After any ending of Jollibee's (night 6 was made sense to all of them except the Discovery ending), the phone guy on night 6 says "Thanks for coming back, our old guy went silent.", meaning you are playing as the previous guard, and the phone guy also says "You already know what to do.", showing that he told the previous guard what to do against the animatronics.

But this is just speculation.