I am changing out an older Toshiba switch with existing DID's and I am changing it to a magix switch w/ DID's and AT&T is sending 4 digits, and the range starts from 1700-1799. I have changed the ext. #'s to start at 1700 but I only have 3 412TDL cards , my problem is the highest ext.# on the switch only goes up to 1735 and some of the customers ext.#'s are ( for example ) 1790 ,when I try to change the ext. # the switch will not let me change an existing # that is not being used