AT&T System 25 - Manual (Page 9)

Ringing Tones
System 25 signals you with four distinctive rings. The rings and
their descriptions are listed below.
INSIDE CALL llllllllllllllllllll (one long ring)
Your call is from a person inside your company.
OUTSIDE CALL llllll llllll (two short rings)
Your call is from someone outside your company.
ABBREVIATED ALERT lll (one short burst of ringing)
This can indicate a call on another line; a busy-to-idle reminder;
a manual signal; or that your telephone is in program mode.
PRIORITY RING ll ll llllllllllllll (two short rings and one long ring)
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.)
On the Selector Console, a steady red light
indicates that a telephone is in use, or a call is
parked on this extension.
On the Selector Console, a blinking red light
indicates an extension that is calling you, or an
unanswered call is returning to you from that
extension.
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