So Brian was supposed to call the invitees to confirm. It would seem likely that'd he'd have made those calls right around the time he was ready to depart the UTS, so as soon as he got away from Clint/Meredith (who were not invited). In my view, this suggests that whatever happened to Brian happened within moments after he was last seen around 2am, and thus quite possibly before he managed to exit the Gateway complex (which Gateway surveillance cam video suggests he did not, and which neighborhood surveillance video also provided no evidence he did).
So, to me, both the phone suddenly going straight to VM, and Brian's having failed to make calls to invitees, hint that Brian may have, very soon after 2am, become unable to make calls, and his phone may have become not able to link up with a cell tower. Scenario A.
Or, he may have decided to blow everyone off and turned his phone off. Scenario B.
The fact that at that moment Brian permanently disappeared points at Scenario A. In other words, whatever factor(s) blocked the signal and blocked ability to make the calls may well also be factor(s) that led to Brian's demise. One potential factor - an accident in the construction area....
Scenario B seems unlikely. Why would he blow off good friends who he evidently wanted to see? And he had a steady GF, who'd gone home for a few days. 2am Friday night, bad time to turn off the phone, cuz, you know what the GF is gonna think .....