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TUCoPS :: Phreaking General Information :: dtmfc.txt

DTMF C is reserved for "Government Functions"?

Subject: Rules/regulations regarding the use of DTMF-C in North America
Category: Science > Technology
Asked by: ttarhan-ga
List Price: $100.00 	Posted: 17 May 2004 19:54 PDT
Expires: 16 Jun 2004 19:54 PDT
Question ID: 347979

Background: We've got custom CPE (customer premises equipment) that
needs to communicate with a special server/switch over the PSTN using
nothing other than DTMF tones.  The connection between the local-user,
the switch and far-end-user is already established and beyond the
scope of this question.

A quick diagram of the telephone connection, with insignificant
components left-out:

[Local User/CPE] <---> [Our Switch] <---> [Far-end User]

These special DTMF tones that communicate information from the CPE to
the switch may be sent in the middle of a conversation and need not be
confused for normal user-generated DTMF tones that might occur in the
middle of a conversation (for example a menu selection in an
auto-attendant).  Also, the tones are not clamped -- that is, the
far-end user also hears the tones.  Most of the time, this isn't an

Problem: We've played with the use of DTMF-A, since it's not normally
dial-able by a user (your phone keypad only has 0-9,*,#).  DMTF-A,
however, causes a problem with Lucent's Octel voicemail system. 
Specifically, if the far-end user happens to be Octel (playing a
greeting, recording a message, or anything else) and it hears a
DTMF-A, it will simply hang-up.  The same is true of DTMF-B and

DTMF-C, however, seems to be safe-to-use.  Octel simply ignores it and
goes on with whatever it was doing.  It's simple enough to switch our
CPE and Switch to use DTMF-C, however, there is one area of concern:

According to Telcordia (Bellcore) GR-1273-CORE, "Generation of DTMF C
has been reserved for various government functions in North America". 
The same document, however, explicitly allows the use of DTMF-A,

This question is regarding what government function DTMF-C is reserved
for, and if the use of it is regulated in any-way.  Also, this
question extends to restrictions that the LECs or IXCs may impose
regarding the use of DTMF-C.

The restrictions may be regulatory or technical in nature.  That is,
the LECs or IXCs may prevent the transmission of DTMF-C, or FCC
regulations may simply specify that it's just not allowed.  Either
type of explanation will be acceptable.  Also, evidence that there's
no problem with using DTMF-C over the PSTN will be considered a valid

Note that simply saying DTMF ABCD were once used for AutoVon IS NOT an
acceptable answer.  This question focuses on the CURRENT implications,
and specifically on why DTMF A, B, and D are allowed while DTMF-C

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