DTMF in call works on some calls and not others. What could the problem be?

There are several factors involved in maintaining consistent in call DTMF.

1. Changing between DTMF methods can cause issues

RFC2833 is the accepted standard by all of the major carriers use it if you can.

SIPINFO is a reliable method if all parties are using it. If it has to be transcoded to inband or rfc2833 then DTMF delays can be introduced. These delays can cause certain far end switched to disregard your DTMF touch tones.

INBAND is the least reliable of the touch tone methods. It requires any switch doing man in the middle or feature processing to convert and re-transmit the touchtone. This can add errors to the touchtone. The issues get worse as latency and jitter are introduced.

2. Phone switches and PBX systems that provide DTMF based feature handling can cause in call DTMF to fail by the time it gets to the far end switches.This is caused by the man in the middle syndrome. It is a case that each switch needs time to process the DTMF to see if it needs to act upon the touchtone. This would be for features like call flash, call transfer, call hold caused by DTMF key presses. This can cause a delay between the last audio packet and the time the DTMF touchtone is relayed to the far end switch. If the delay goes above a given threshold that touchtone digit can be thrown out or ignored by the far end switch.

Sonus switches are know for this issue as they require a delay not exceeding 100ms. Asterisk based switches can often exceed 150ms when doing feature based processing. There are other switches as well that exceed this Sonus requirement.

Sonus switches are used by many of the large carriers thus causing a proliferation of incall DTMF touch tone issues. There is an on going effort by many phone system developers to over come the Sonus DTMF delay requirement.

For Astrisk based system users there is a tracker issue you can reference.

https://issues.asterisk.org/view.php?id=15642

For other phone systems he issue you want to look for is what is the delay between the last rtp audio packet just prior to a DTMF event                     packet being transmitted. If it exceeds 100ms and a Sonus is acting as the far end switch then you will likely have issues. Have your vendor see             if they can add the ability to send a blank rtp audio packet just prior to sending the DTMF event packet. This should cause the far end switch               to reset it's delay counter and the issue will likely go away.

3. Jitter and Latency are another factor in DTMF failures. If your connections lose too many of the DTMF packets the far end switch will not have           enough information to recreate the DTMF touch tone.

Sonus is a registered trade mark of Sonus Corp.

Asterisk is a registered trade mark of Digium corporate.

This is a draft document written by Bryant Zimmerman (ZK Tech Inc.)

It contains a limited subset of information on this issue and should not be construed as a complete authoritative document on DTMF issues. The information in the document is offered “As Is” with no expressed or implied “Warranty”. © ZK Tech Inc. 2011

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