Jump to content


Vodia PBX

Recommended Posts

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:



o=root 971198566 971198566 IN IP4


c=IN IP4

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




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.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...