[Asterisk-Users] Polycom DTMF

[Tom Samplonius]

On Thu, 24 Mar 2005 09:36:21 -0600, David Gomillion
<dgomillion at eyecarenow.com> wrote:
> Problem:
>    Polycom SoundPoint IP phones (running SIP) ceases to send DTMF that
> Asterisk can detect and use.  It worked in 1.0.5, but has not worked
> since.  This has been verified on SoundPoint IP 300's and SoundPoint IP
> 600's.
> 
> Workaround:
>    It used to be that for DTMF to work, I had to set the mode in
> sip.conf to "inband".  Without making any configuration changes on the
> phones, I changed the DTMF mode to "rfc2833".  The DTMF is recognized.
> No reboot to the phone is necessary, and remember that you can reload
> the sip configuration with a reload in Asterisk, meaning your PBX
> doesn't have to be restarted either.
> 
> Discussion:
>    This is probably not the "right" way to fix this, as Polycom's
> configurations, by default, will "encode DTMF in the active RTP stream".
...

  No, it is probably the right way to fix it.  RFC2833 is the
preferred way of transporting DTMF.  RFC2833 encodes the DTMF into the
RTP packets as distinct messages.  In fact, RFC2833 is an enhancement
to RTP to carry DTMF.

  Inband signalling also puts the DTMF into the RTP too, but into the
media itself.  That means the DTMF is encoded as sound.  Inband DTMF
should be avoided whenever possible.   On low-bandwidth codes, it is
very easy for the frequency of the sounds to be altered slightly so
the tones are not recognized.  Not to mention all of the issues around
the lenght of the tones, leading to missed digits, or doubled digits.

Tom