I have incoming calls hitting a LDN which is on a 5540 and 5340. I wanted to add a 5312 to the incoming call LDN. I am having trouble finding this key to add. Can you put a LDN on a 5312. What other way can this be built?
No... In order to place an LDN key on a set on the SX-200, it must be an attendant console, or a set configured as a sub-attendant, and that is not possible for a 5312 set, only 5340 and 5330 (on later softwares) or high end digital sets (4050) can be sub-attendants.