SIT's are the three sequential tones usually followed by a recording like "We're sorry, your call did not go through as dialed..." Usually they are not very helpful to the individual.

SIT's are also used by Telemarketing equipment. ADAD's or Automatic Dialing Anouncement Devices listen for SIT's and they will parse the dialed number from its calling database as it is not working (this is how Radio Shack's Telezapper works. A clean SIT recorded to an answering machine can be just as effective).

Encoding

SIT's do have an encoding scheme. The duration and frequencies used in this scheme provide 32 different possible encodings.

Tones 1 and 2 consist of a Low or High frequency. The third tone is always 1776.7hz for easier decoding.

These frequencies should held within +/- 0.2%.
The duration of the first and second tone is either short (274ms) or long (380ms) +/- 5%.
The duration of the third tone is always 380ms.

The duration of silence between tones is not part of the encoding, but it should not be greater then 4ms. Interuptions of a tone should be less than 1ms for optimal decoding. The amplitude variation among the 3 tones should not be greater than 3dB.

Announcements

The duration of silence between the third tone of the SIT and the prerecorded message should be no more then 300ms, and 400ms for localy recorded messages.

The amplitude of the SIT should be 2dB lower than the annoucement.

Announcements try to identify to the user the condition and the respective action to be taken. Announcement types are as follows:

No Circuit (NC)
ReOrder (RO)
Vacant Code (VC)
Intercept (INT)
Inneffective Other (IO)

NC
A No Circuit applies when there is a failure to find a trunk or to calls affected by Network Management Controls. The call should be tried again later.

RO
The ReOrder is due to internal switching blockage, failure to recieve wink on an interoffice call attempt, insufficient or mutilated digits, failure to find an available resource, interoffice signalling link failure, all announcement trunks busy, switching equipment congestion or protocol errors. The call should be tried again later. (A ReOrder tone resembles a busy signal, but is twice as fast (120 interuptions per minute.))

VC
Vacant Code may indicate any of the following: Vacant Code, Unauthorized CAMA '1' or '0' plus unauthorized code (??), nonworking 911, out of area, terminating end office or tandem recieves digits for nonsubtending code, no route to specified transit network, or no route to destination. An announcement should contain instructions.

INT
Intercept applies when the number has been changed, vacant number, disconnected number, non-working number, temporarily suspended service, incoming call restriction.

IO
Ineffective Other applies under the conditions; prefix or access code dialing irregularity, improper initial coin deposit, screened access denial, or any dialing irregularity.

SIT Coding Assignments