DTMF through SBC issues?

[dandenson]

I've got some Yealink T4x series phones behind the 3CX SBC. I can't dial remote DTMF. If I do the same test on the same 3CX (hosted) instance but not through the SBC, works fine.

Anyone else experience this? Is there something in the SBC I can change?

Provisioning has the phones set to RFS2833 (or rather, it's blank so the default is used). In asterisk this was solved by switching to 'info' and having the pbx generate the tones but I can't find any information on 3CX being able to do this.

 

[Dominik Oelmaier]

Hello,

It may have something to do with the DTMF Method that your sets are using. Codec should not matter unless you are using DTMF audio only with a lower rate Codec. In the case of a VoIP provider you should be sending DTMF as a "message", and possibly as audio. Many sets have an "auto" setting.


best regards,
Dominik Oelmaier

 

[eddv123]

Codec could matter but only in the event that you were using a compressed codec (such as G729) in conjunction with DTMF INBAND which includes the DTMF digits in the same stream as the audio (unlike INBAND RFC 2833 which puts DTMF is a separate stream or the out of band SIP INFO where DTMF tones are taken out of the media RTP stream and transmitted using the signalling protocol – i.e SIP.).

A lot of SIP handsets still support INBAND DTMF but it is not often used and would not be recommended. RFC 2833 is normally the default

I would run a Wireshark trace to see what is being generated and at which points the DTMF is present (so you can pin-point where the weak link in the process is) you will either see the DTMF digits being transmitted in the RTP packets or via SIP INFO quite obviously - unless of course you are Encrypting the traffic.