[asterisk-users] Double DTMF digits

[Ken Leland III]

When dtmfmode is set to inband for SIP, and i originate a call from sip 
out to the PSTN, I can hear the DTMF digit twice in the audio stream. 
Once very briefly and once for normal duration.

Our Theory: While Asterisk is parsing the DTMF, for a fraction of a 
second, while the end user generated DTMF is being detected, the DTMF is 
passed inband. Once the DTMF is detected Asterisk silences it and 
regenerates it. Sensitive machines like auto attendants pick up both the 
brief end user generated tone as well as the full length asterisk 
generated tone and ultimately perceive each digit twice.

Is anyone else experiencing this?

I have reproduced this in an environment
    * with one asterisk server that is both the feature server and the 
media gateway, and is timing off of network T1s
    * with two servers, one feature server (timing off of ztdummy) and 
one media gateway (timing off of network T1s) using IAX as the inter 
asterisk protocol

It is pretty easy to reproduce:
-Dial a PSTN number(like your cell) from a sip phone using inband DTMF, 
and configured in asterisk sip.conf with dtmfmode=inband.
-Answer the PSTN end.
-Press and hold a digit on the sip phone. On the PSTN phone you will 
hear a very brief, end user generated, tone.
-Let go of the digit on the sip phone. On the PSTN phone you will hear 
the asterisk generated tone.

Can anyone else hear the brief initial tone?  Any help is greatly 
appreciated!

There is a good wiki article on DTMF:
http://www.voip-info.org/wiki/view/Asterisk+DTMF

and there has been a thread about it on this mailing list:
http://lists.digium.com/pipermail/asterisk-users/2007-March/181461.html

We have opened a bug report:
http://bugs.digium.com/view.php?id=9382

Some System Specifications:
Asterisk 1.2.14
Digium Wildcard TE405P/TE410P
CentOS release 4.4 (Final)

-- 
Ken W. Leland  III
Engineering
k3leland at monmouth.com
Monmouth Telecom
10 Drs. James Parker Blvd., Suite 110
Red Bank, NJ  07701
732-704-1000  X283