[Blink] Blink - DTMF problem

Andreas Sikkema asikkema at unet.nl
Tue Dec 24 01:12:48 CET 2013

Jose Pablo,

> Calls do not fail due to DTMF mismatches, however, codec mismatches are
> fatal and will prevent a call from completing. DTMF methods and codecs are
> different things.

One wouldn't think so, but it just so happens that I've seen just such
an occurrence a few weeks ago. We have one Cisco gateway still running
an older (12.4T) IOS version that by default expects payload type 96
for one of its proprietary fax implementations. This can be fixed, but
so far we'd never had any problems. When a call is sent to the Cisco
with a default config using RFC2833 using pt 96, the gateway will
completely ignore all RFC2833 entries and only reply with an audio
codec for the audio media stream.

Since a few weeks one of our carriers appears to have connected to
another carrier using SIP. This other carrier has a really funky SIP
platform that does lots of weird things, including rejecting calls
when it receives SDP without RFC2833 contents.

So yes, calls fail due to funky "DTMF" interworking. My mind was off
the rails for a few days trying to comprehend it all, but that is the
reality of running a SIP platform connected to things other than the
relatively sane SIP implementations us Open Source people expect.

Remind me to never try to explain some other weird stuff I've seen
from so called "world class" enterprise SIP implementations.

Andreas Sikkema

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