I am working on a GTE Executech 6x16 with 6414 sets (some 6414L, some 6414AB). A few sets were old and I replace them. On the old phones, the caller could hit 'INTC', dail the 2-digit intercom #, and be on the speaker of the called phone. After replacing the sets, when intercom calling using the same procedure, the call just rings on the called set until someone physically picks up the call. I've tried 'INTC # 2' and it still does not work. Some co workers seem to think there is a phone compatability issue; the new phones, while the p/n is the same, it isnt the same. Is this true? How do I rectify this?