I think it depends on the school.
That being said, the Dec.1st date is a regulation or law if you will saying the NO school is allowed to send out acceptances before then. There is no such rule for rejections (as some of us have unfortunately found out) so I don't see why any school would wait until dec1st to tell someone they'd been rejected once the adcom had already made that decision.
It'd be especially cruel if they did it by phone! Lol. See an unknown number on dec1 and get all excited and someone says "JUST KIDDING, you're out!" - michael scott moment for the office lovers on SDN.