AT&T ATT 993 - Manual (Page 55)

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Caller ID
Call ID operation
Call waiting
If you subscribe to the caller ID with call waiting service
and there is a call waiting call, the screen displays call
information for the call:
Press FLASH to access the call waiting call.
To switch back to the original call, press FLASH again.
.
2.
Message waiting and NEW CALL lights
There is a NEW CALL light for each line. A NEW CALL
light stays on when there is unreviewed call information
on that line. If you subscribe to a voicemail service, a
NEW CALL light flashes when there are unretrieved
messages waiting on that line. The screen also displays
information about the number of new calls for each line.
2:08 PM 2/26
L : 22 New Calls
L2 :  New Calls