asterisk users - Jan 2005 - IAX line gets 'Hungup' after period of silence

Hi.

[I asked a similar question a while back, but unfortunately wasn't
around to reply to the responses, so sorry if you experience any deja
vu.]

I have a * server acting as an IVR system. The calls come in via IAX.

After a period of about 40 seconds of silence (either waiting for the
caller to dial an extension, or with the audio paused in
controlplayback), the call hangs up. All I see in the CLI is "Hungup
'IAX2/voipuser@...'"
I know it isn't a timeout in * because they give a more useful message
- even so I've now set the absolute, digit and response timeouts to be
large, and created t and T extensions, but it's still happening!

I have 3 inbound phone numbers acquired from 3 different places, but
this happens on all of them.

Is there something I could have overlooked? Or is it more likely down
to a firewall or something at the ISP dropping the connections after
so many seconds of no packets?

Help me please :-)

Regards,
David