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
issue.

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-D.

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,
DTMF-B, and DTMF-D.

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
answer.

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
isn?t.

---

Clarification of Question by ttarhan-ga on 17 May 2004 20:01 PDT

Simple typo: "need not be confused for normal user-generated DTMF"
should read "must not not be confused for normal user-generated DTMF".

Hello-

Thank you for your question.

If you require any further clarification, please ask before rating this answer.

The direct answer to your question is: while connected, there should
be no problem using DTMF-C on a regular phone connection.  It is "just
another tone." Frequencies of all sorts are generated during typical
phone conversations.

The reaction of both the FCC and Telcordia can be summed up in a
statement by the the director of DeviceWare for Telcordia:

"I know of no legal restriction barring the use of DTMF C. It is in
use in private and public networks, both nationally and
internationally."

According to Fact Index
http://www.fact-index.com/d/du/dual_tone_multi_frequency.html
"The US military also used the letters, relabled, in their Autovon
phone system. Here they were used *before dialing the phone* in order
to give some calls priority, cutting in over existing calls if need
be. The idea was to allow important traffic to get through every time.
Pressing C, Immediate, before dialing would make the switch first look
for any free lines, and if all lines were in use, it would hang up any
non-priority calls, and then any Priority calls. While the Autovon
phone system no longer exists, their original names were Flash
Override (A), Flash (B), Immediate (C), and Priority (D). Pressing one
of these keys gave your call priority, over-riding other conversations
on the network. Flash Override is the highest priority."

While connected, however, DTMF C is generated by the crossing of two
frequencies that could be theoretically generated by music making
equipment over a conversation.  There's no barring or censorship of
this sort of thing.

To elaborate on the GR-1273-CORE statement, ?Generation of DTMF C
has been reserved for various government functions in North America?,
the rep from Telcordia says:

"At the time ADSI was first being developed, Bellcore issued a draft
of the document for Industry comment.  One or more comments came back
indicating that DTMF C was in use for applications/services that may
run concurrently with ADSI calls/sessions.  To avoid unwanted
interactions, we chose to design around DTMF C ... I have never heard
of any state or PUC regulations on limiting the use of DTMF signals."

Search strategy:
  contact FCC and Telcordia
  google: dtmf
  google: dtmf c

Looks like you put a good amount of time into this.  Very impressed.  Excellent!

Thank you very much for the kind words, rating, and tip!  Glad to be
of help.  Please stop by again!

jbf777