Android NGN stack: AV connection dropped after 40s with Mobotix intercom

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I use the Android NGN stack to connect to a Mobotix T24 camera, and I notice 
the connection gets dropped after more-or-less exactly 40 seconds. This happens 
both with IMSDroid and my own integration of the NGN stack, and it does not 
seem to matter what codecs are used.

What steps will reproduce the problem?
1. Set up the Mobotix M24 to work as a SIP server
2. Register the IMSDroid application with this server, using a valid account
3. Place a call directly to the IP, in our case using the URI 
"sip:10.4.1.4.81:5060"

What is the expected output? What do you see instead?
- The call connects correctly, showing both audio and video.
- Connection gets dropped after 40 seconds,
  - showing a 'call terminated' notification in IMSDroid, and
  - the camera seems oblivious to the termination, but cancels the connection after a timeout of five seconds (by default), since it doesn't receive any data anymore.
I have attached a Wireshark trace of the interaction.

What version of the product are you using? On what operating system?
I have used imsdroid-2.0.509.apk, and have tried it with the NGN stack which is 
included in IMSDroid r512 .
We run it on an Acer Iconia Tab A500, running Android 3.2.1.

Please provide any additional information below.
I have noticed that the Mobotix camera uses a LinPhone stack.

Original issue reported on code.google.com by Angelo...@gmail.com on 29 Aug 2012 at 3:58

Attachments:

• mobotix_trace

[GoogleCodeExporter]

By now, I have not been able to find out much more. I tried a number of 
different audio codecs (Alaw, Ulaw, GSM are supported by the camera) and video 
codecs (H263, H263+ and H264), but the effect is identical: after 40 seconds, 
the connection gets terminated, and the camera detects this after a 
data-timeout.

Have I missed some option? Is the camera not behaving well?

Original comment by Angelo...@gmail.com on 11 Sep 2012 at 7:42