asterisk users - Jan 2010 - Temporary loss of audio on all SIP channels

Hi, I'm trying to diagnose a particularly elusive problem, and am
wondering if anyone else here has seen anything similar and can offer
any ideas.

I have a conference bridge running Asterisk 1.2.32 (with slight mods),
in a colo talking via a LAN to an ITSP using SIP/RTP. It is dedicated
to a single customer.

On several occasions over the last few months, the customer has reported
instances of all conferences in progress losing audio. They described it
as dropped calls, but we didn't get a sudden batch of simultaneous
hangups. It seems more like the lines went quiet, and the callers then
took varying lengths of time to hang up and try again. The hangups
were logged as normal clearing.

When the callers tried again, it is reported that although the call was
answered, they did not hear anything (they should have heard a greeting
requesting a code - not directly from Meetme, but from an AGI - and the
playback of this greeting was actually logged as normal). This
continued over repeated attempts for a period of several minutes, after
which the problem resolved itself without intervention; callers then
successfully heard the greeting, entered their code and were directed
to the Meetme application.

Nothing appears in the asterisk logs to give any clues. Asterisk is
running at verbose level 3, and the "full" log is set to log all
classes
of message.

Because of the lack of logged errors and warnings, we believed this
problem to be at the ITSP, although they couldn't offer an explanation.
So we changed ITSP. Unfortunately, the same behaviour has now been seen
once with the new ITSP. So it makes me wonder whether it is an obscure
problem in the Asterisk box itself.

I have a continuous tcpdump logging SIP packets. Unfortunately, it is not
practical to do the same with RTP packets, due to the huge volume of data
that would be generated.

Does this problem match anything seen by other users? Any ideas?

Thanks in advance,
Tony

-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
Play: tony at mountifield.org - http://tony.mountifield.org