Yes, you read right: Fritz is the Phone Guy. Why, you ask? It's very simple. First off, Phone Guy told Jeremy that A-Jeremy is the second night guard (on Night 1), and B-That when the place eventually reopens, Phone Guy would take the night shift himself (On Night 6). Then, we meet Fritz Smith on Night 7, who A-Doesn't get a phone call, yet knows the mechanics very well, and B-Is Employee Number 3, right after Jeremy, and gets fired on the very first day. Given that the reasons he got fired were all things that HE did and not that the place was, well, closed down, the place must have reopened, which means Phone Guy took the night shift himself. And who's taking the night shift here? Fritz Smith. Fritz is the Phone Guy..
P.S-A common debunk to this theory is that there was one more event before the place closed down-A birthday. However, it was stated that that would be the next day, as in, before the next night. Therefore, it must have closed down before the next night, and reopened, meaning that Phone Guy takes the night shift himself, who is Fritz.