I’d like to know who he was talking to on the phone. Could be he had called his mother and told her what he was going to do. Not much she could do within the timeframe even if she believed him. The police were able to contact his parents rather quickly so it could be all they had to do is check the last number called.
I personally think it was a ploy to look like he was taking a phone call so he could exuse himself out the emergency exit. My guess it that no one was on the other line. In his planning, he probably wondered how he could leave without drawing susupicion and figured a phone call would do that.