The NT-1 on an ISDN line typically has lights for power, U Loop Back, and S/T&U. These lights can be used to troubleshoot your ISDN line. Below is a table showing varing NT-1 light states:
Normal Light States
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Light State Reason Check
----------- ------------- ------------------------ --------------------
Power On Power is on N/A
U Loop Back Off Line isn't being tested N/A
S/T&U Off Line is ready for use N/A
Other Light States
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Light State Reason Check
----------- ------------- ------------------------ --------------------
Power Off There is no AC power Power cord, AC wall
jack.
U Loop Back On/blinking Line is being tested Contact local phone
company to see if
line is actually
being tested.
S/T&U Fast blinking Line is attempting to If light then goes
(many blinks/sec) communicate with central out, the line is
office ready to be used. If
the light stays on,
then check both
cables coming out of
the NT-1. If the
problem continues,
contact your phone
company.
S/T&U On steady Line is not hooked up Check cable going
properly from wall jack to
the NT-1.
S/T&U Slow blinking Line is not hooked up Check cable going
(1 blink/sec) properly from NT-1 to the
ISDN device.