I really don't see any particular mystery around the 911 call.
A friend came over near noon, let himself in, or rang the bell and was let in by the surviving roommates.
He/she/they went upstairs to make a coffee or whatever, saw the "unconscious" person lying on the floor, and immediately called 911.
The visitor didn't have his phone handy (maybe still downstairs, in a jacket pocket downstairs, maybe in his car), a roommate did have it with her, so the visitor called 911 from her phone.
Not feeling at all this caller is the killer personally.
eta, and not named for privacy....didn't the daily Mail out the two roommates names? Not LE I believe