AT&T System 25 - Manual (Page 10)

Ringing Tones
System 25 signals you with four distinctive rings. The rings and
their descriptions are listed below.
Your call is from a person inside your company.
Your call is from the attendant or someone outside your
company.
This can indicate a call on another line; a busy-to-idle reminder;
a manual signal; or that your telephone is in program mode.
You are being alerted that your request for Callback can now be
completed; or a call is being set up remotely for your phone via
a data feature known as Third-Party Call Setup.
Lights
A steady red light indicates the line you should
use when you lift the handset.
A steady green light indicates the line or feature
in use.
A slow blinking green light indicates an
incoming call.
A fast blinking green light indicates a call on
hold or a queued call. (An intermittent blinking
green light indicates a call held temporarily while
being transferred or conference.)
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