ADD is automatic digit display but if your on the line and a second call comes in, it rings on the other line but you don't see the calling info. But with TDD you see the info on the second line while you are talking on the first line.
ADD (Auto Digit Display) allows Caller ID to be displayed automatically when a call comes in on your KEY 0 call appearance. TDD (Tandem Digit Display) allws Caller ID to be displayed on ANY key that a call is ringing in on. In fact, it also displays CLID for keys programmed NOT TO RING. Thats kind of a bummer for a secretary that has like 80 SCN's, but thats that pesky Nortel Logic.
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