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DTMF mangled on outbound

Discussion in '3CX Phone System - General' started by jtbool, Dec 16, 2016.

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  1. jtbool

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    Hi,

    I'm hoping that someone might have an idea about a DTMF problem I'm getting ever since I switched to 3CX...

    Situation is this:
    Cisco SPA504G phone, set to send DTMF as AVT (RFC2833). 3CX PBX, current version, Debian OS. VOIPfone provides my outbound trunk, expects DTMF as RFC2833. So, the whole chain of DTMF should be RFC2833 until VOIPfone offloads the call to analogue POTS on the other side.

    If I dial out by connecting the phone directly to the trunk, all is well, so it looks like the phone generates good RFC2833 and the provider handles it and can turn it into analogue DTMF reliably.

    If I dial out via the 3CX PBX, DTMF digits are often doubled-up and occasionally missed, so that dialing 1234567890 might be received at the other end as 122456778900. It appears from this, that the problem is either (1) 3CX not handling the RFC2833 messages from the phone properly or (2) 3CX not passing them on in a way that's acceptable to the provider's system.

    Are there any configuration settings that I should tweak in 3CX? Anywhere I can debug what 3CX receives from the phone and what it sends out on the trunk?

    Thanks!

    Jeff
     
  2. jtbool

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    Update... the problem seems to be between the Cisco and 3CX PBX.
    I tried two more tests:
    (1) a Polycom phone - DTMF worked fine with 3CX, although I can't tell from its config, whether it's using RFC2833 or something else.
    (2) switching the audio codec on the Cisco - going from G711u to G722 as default seemed to improve things, interestingly.
    Also checked the 3CX settings for the extension - DTMF is set to RFC2833 and there are several codecs in common between phone and PBX so no obvious reason any for problems. I've made G722 the top codec choice on both ends.
     
  3. leejor

    leejor Well-Known Member

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    I've seen the double digits before using a gateway. It seems to be caused by the audio DTMF being sent at the same time as DTMF being re-generated "down the line" by SIP messages. The fact that things improved when you went away from g711 may back this up. Attempting to send DTMF as audio, over a lower bit rate codec, usually has issues, so when you had it set to use g711, the audio DTMF probably got through as well as the DTMF SIP messages, giving the double digits. You may need to check the phone settings to see if audio DTMF can be completely suppressed.
     
  4. jtbool

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    Thanks, I'll look into that. I'd hope that Cisco has the sense to kill the analogue or to not send it at all when it's using RFC2833, but being Cisco, it probably has an obscure setting somewhere for it!
     
  5. leejor

    leejor Well-Known Member

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