SIMPLE, INSTALL AN ATI-D AND ADD JACK NEXT TO EXISTING PHONE. RENUMBER DIGITAL EXTENSION AND MAKE NEW ATI-D PORT THE ORIGINAL EXTENSION. CONNECT A HORN/STROBE TO ANALOG JACK WITH ORIGINAL EXTENSTION FOR INTERCOM CALLS. PROGRAM NEW DIGITAL EXTENSION FOR INCOMING CALLS. AND PROGRAM A PICK-UP BUTTON ON DIGITAL EXTENSTION SO YOU CAN ANSWER INTERCOM CALLS. THIS WILL GIVE YOU EXACTLY WHAT YOUR LOOKING FOR.