Didn't Pacino lose his cell phone in one scene, and then answered a call on it in the next, or he had borrowed a cell phone and lost that and then answered a call on it, or something like that.. I didn't see anything about that in the Goofs section so maybe I just imagined it..
Revenge is a dish that best goes stale.
he accidentally dropped his cell phone and it broke so he borrowed his student's. he did give his assistant his new borrowed phone number. but then he did seem to get many calls on the borrowed phone from other people (like the person threatening him), even though no one else knew what number to call him at. at the end he purposely breaks the borrowed cell phone.
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I thought it was gonna be a clue. Up to when (spoiler) the principal calls him, I'd assumed his assistant was patching all the calls through. I think most of the time, the assistant spoke to let him know who was on the line (and to find out if he wanted to take the call).
Of course, when the principal calls (and obviously the killer is making her do it) the assistant is in the room. But only Kim and the assistant knew that he was using Kim's phone. So that seemed like a clue that one (or both) of them was in on it.
Even when the killer died, I assumed that the final twist would be that he'd realise that Lauren wouldnt have known which number to get him on.
But at the end of the day, it seems it wasnt a clue at all. So presumably EITHER the assistant set up his calls to automatically divert OR he had other staff in the office who patched the principal through (without speaking to ask their boss if he wanted to speak to her).
There were a fair few deliberate red herrings, so I think this probably was deliberate too.
Eg
The taxi driver was insignificant. I was expecting one of the photos of the prison visitors to be of him.
The suspicious acting concierge was just creepy, and not a conspirator.
The student who forged the signatures to get into prison wasnt in on it either.