DTMF with SCCP and SIP phone

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Feb 13th, 2009
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I have a problem with DTMF with SCCP on Phones 7960 and a trunk SIP toward Service provider

1) SCCP Phone 7960 --> CUCM 5.1 --> SIP Trunk --> Service Provider --> IVR

2) SIP Phone 7960 (fmw PC030301) --> CUCM 5.1 --> SIP Trunk --> Service Provider --> IVR

In first scenario the digit sent to the IP Phone are not recognized by IVR in the second scenario yes.

Now, Both SCCP and SIP support RFC2833 - NTE this means that the dtmf are sent inband, into the RTP flow.

Why in the first scenario not working only with SCCP ?

The SIP trunk is without flag "MTP required" and the chioce about the DTMF signaling Method is left to

"no preference".

Is not possible to flag MTP because the customer has restricted request and this option is not conteplate.

Eventually i can move to other IP Phone.


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paolo bevilacqua Fri, 02/13/2009 - 06:34
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Hi, with cisco phones, DTMF is never sent inband. rtp-nte means exactly that are not sent inband. Your ITSP must support some type of dtmf relay and you configure it in the SIP trunk.

Else, you will never be able to send digits.

m.cucchi Fri, 02/13/2009 - 06:48
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Sorry just to understand .... RFC 2833 mean inband ?

The IP phone SCCP support RFC2833 and SCCP DTMF, SIP phone support only RFC2833.

In SIP trunk we do not have any preference on this.

Another update .... with 7961 no SIP and no SCCP work with DTMF.

Firmware problem ?


paolo bevilacqua Fri, 02/13/2009 - 07:03
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Hi, rfc2833 does not mean inband.

Sorry, I don't know what the cause of the problem, what I can tell you, DTMF is never sent inband with cisco phones.

m.cucchi Fri, 02/13/2009 - 07:17
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Soory i have search in RFC2833


but seems that the DTMF are sent into the rtp payload.

This memo describes how to carry dual-tone multifrequency (DTMF)

signaling, other tone signals and telephony events in RTP packets.


Nicholas Matthews Fri, 02/13/2009 - 11:35
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Hi Massimo,

To clear some things up:

Out of band: This is normally SIP NOTIFY, H245-alpha, or H245-signal. Sometimes rtp-nte is considered out of band, sometimes not. More likely - not.

RFC 2833: It does an out of band style in-band. This means since the DTMF is in the audio stream, none of the signaling devices are notified of the DTMF, which it is more commonly referred to as in-band.

In-band: Actual DTMF frequencies in the audio stream.

Newer Cisco IP phones will support using RFC 2833 directly from the phone. You may want to consider this on your SCCP phone if you're having troubles.

As well - if you do not have an MTP required for your outgoing SIP trunk, SCCP keypad messages will not be converted to RFC 2833.



jonesbrett Thu, 02/26/2009 - 03:16
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Hi Massimo,

I had the same problem with the SIP trunk from the ITSP terminating on a H323 gateway. I set up a SIP trunk between CUCM and the gateway which resolved this problem.



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