[VoiceOps] PRI loss/level settings - DTMF issues

Jay Hennigan jay at west.net
Fri Mar 18 14:32:33 EDT 2011

We are having some issues with doubled DTMF digits from a customer
dialing into conference bridges, etc.

Scenario: Shoretel PBX  -> PRI -> Adtran TA900 -> SIP trunk.  We are
using G711/RFC2833.

The problem is intermittent, about 5% of calls or less, read-back of
received digits shows occasional doubled digits, almost always DTMF "1".

We were able to duplicate the issue with the Shoretel system going out a
conventional TDM PRI as well, but the customer and the conferencing
vendor and the customer tend to blame the problem on "the VoIP system".
  It is very intermittent and DTMF works fine most of the time.

The Shoretel itself sends only a short burst out the PRI regardless of
the length of time the DTMF key is held.

Comments from voice users on the receiving end are that the Shoretel
caller's voice seems "hot".  The Adtran has settings to adjust level on
FXS but not on PRI.  The Shoretel does have adjustments, presently set
to 0 dB.

It is my recollection from the old Bell/Mitel days that there should be
padding on T1/PRI trunks but I can't find a spec.  From memory I want to
say -6 transmit and -3 receive but Google is failing me on finding this
documented.  Is there a standard loss/level best practice for this?

The fact that the issue occasionally occurs on the TDM trunks leads me
to think that the SIP RFC2833 conversion is not to blame and that
perhaps there is acoustic coupling between the tone played out the
phone's speaker being picked up by the microphone that padding would help.

Any suggestions would be appreciated.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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