Did Holmes do something to keep the door ajar, should then be asked. Taking a true phone call might involve going to the lobby or maybe the bathroom, but leaving the area controlled by the box office seems a bit much. If Holmes could shut the door completely and come back, the door was then not secure and anybody could walk in.
I thought I read that witnesses said he ‘propped’ the door open. Now whether that mean just sticking an object so it doesn’t close or taping the latch so it doesn’t latch, Idk. Witnesses did claim that the door wasn’t shut completely though.