Abstract: A key telephone system having multi-number plan for assigning different number plans to a general telephone set and a key telephone set in a key telephone system, wherein the different number plans control performance of same function. A central processing unit performs the functions designated by the multi-number plan in response to an input from the dial keys of one of the general telephone set and the key telephone set. The central processing unit check an extension circuit to determine when a receiver of one of the subscriber extensions has been lifted, and when a receiver of one of said subscriber extensions has been lifted, determines whether the subscriber extension having the lifted receiver utilizes a first number plan or a second number plan by searching a number plan designation table for a position of the subscriber extension having the lifted receiver.