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DTMF


Vodia PBX
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If DTMF does not work, there is most probably a problem with the out of band codec negotiation. There is still a lot of equipment out there which does not properly advertize the RFC 2833 codec, which is frequently leading to problems with the DTMF.

 

The SDP attachment of the INVITE message should contain something like this:

 

v=0

o=root 971198566 971198566 IN IP4 192.168.9.254

s=call

c=IN IP4 192.168.9.254

t=0 0

m=audio 61434 RTP/AVP 0 8 9 2 3 18 4 101

a=rtpmap:0 pcmu/8000

a=rtpmap:8 pcma/8000

a=rtpmap:9 g722/8000

a=rtpmap:2 g726-32/8000

a=rtpmap:3 gsm/8000

a=rtpmap:18 g729/8000

a=rtpmap:4 g723/8000

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-16

a=ptime:20

a=sendrecv

 

The type "telephone-event" tells the PBX that the codec 101 is used for RFC 2833 DTMF. If that codec is present, then the PBX knows where to search for out of band DTMF codecs.

 

If that codec is missing, there is something wrong with the device or service provider that tries to talk to the PBX. Usually devices have an option to turn this feature on.

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