Every originated call shall result in one of the following:
A. The call shall progress to the number dialed, the calling customer shall receive an indication of ringing, and the called telephone shall ring;
B. The call shall progress to the number dialed and, if busy, the calling customer shall receive a busy signal;
C. The call shall progress only part of the way through the switch train and, having reached an overflow condition, shall receive an appropriate overflow signal, which can be readily differentiated from a subscriber busy signal;
D. A call to vacant code level, code, or subscriber number shall receive an adequate and appropriate intercept where facilities permit.