"Hello? Hello...uh...what on earth are you doing there, uh didn’t you get the memo, uh, the place is closed down, uh, at least for a while."

Why would he be calling except to get a hold of someone. he surely wasn't expecting you to be there. So then, the question remains, who was supposed to receive that call. as phone guy said in the previous night, the building is on lock down. No one, including previous employee's, should have been allowed back in. Our character had to have crossed some sort of police tape or barrier, as there is an investigation going on.

so our character is there ilicitly, yet the company still pays him. As cheap as Fazbear Entertainment is, I doubt they'd pay you for coming on a day you're not scheduled, unless someone from the company asked you to be there, someone who could approve you being paid for that day, especially overtime.

So that leaves some poor payroll schmuck, phone guy himself (who seems to handle the hiring/instructional jobs) or the CEO of the company, who could definitely approve you being there. I could speculate all day, but it's clear that someone wanted us to be there, and that someone was probably who phone guy was trying to contact.