Solution:
After obtaining the gold key from the kitchen (Achievement: Creepers), return to the east foyer. The door to the phone room is different for each playthrough. It is either of the second doors from the left in the east foyer on the top or bottom floor. If the door is still locked, check the second floor. Once inside, Mary Barrows will offer you a drink. This will only happen if you haven't found Walter Simpson's corpse (Achievement: A Great Father). If Mary does not show up, leave & enter again until she does.