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Walkthrough Wednesdays

Jabber and PSTN SIP carriers/ITSP

Paul McGurn
Frequent Contributor

Figured I would share my thoughts on this.

I was recently doing some testing with an ITSP (SIP PSTN provider), and was having trouble getting Jabber to connect. 

CUCM would pass back "488 Not Acceptable Media" for any call Jabber tried to place to the SIP trunk that ties our CUCM to our CUBE.

The root cause is video.  We configure a Region for our SIP Trunk Device Pool that disallows video by setting allocated bandwidth to "None".  But, on our SIP trunk, we did not have the "retry video call as audio" checkbox set.  It seems that Jabber ALWAYS sends the video bits as part of the SIP INVITE it sends with an outbond call, regardless of the setting you have configured in Options > Calls.

Media Description, name and address (m): video 28726 RTP/AVP 126 97

This is not a bad thing, as it's more convenient to use Early Offer for this type of media than it is to have to do a reINVITE-by-default when the clients want to use video.  But, you need to configure CUCM defensively when dealing with Jabber calls to SIP providers outside your.  The working checklist I now have is:

  1. Configure a special Region for your ITSP trunks, and set the video bandwidth to None for all inter-region connections.
  2. Set the "retry video calls as audio" flag on the SIP Trunk
  3. Most ITSP's require early offer, so set the Early Offer support for voice and video calls (insert MTP as needed)
    1. Having MTP's shouldn't be needed as long as your end-to-end codec support matches Jabber's, and your media doesn't need to be transcoded.

The gotcha, is that if you actually have a 3rd-party SIP provider that supports video, you are not going to be able to use the same CUBE for both video and non-video calling.  This is because CUCM won't allow you to create more than one SIP trunk to the same gateway device.  Realistically, Cisco expects you to use the Expressway servers for 3rd-party video calling, so I won't go into that end of it any deeper.

I'm testing all this on CUCM 9.1, IOS 15.3 CUBE, and Jabber FCS 9.7, not sure if anything changes in later versions of CUCM/CUBE.

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