Hi Peter, you would not be trying to connect to BT by any chance?
I'm working an a similar problem. Have a look at
http://www.blokland.co.uk/Kees/docs/e-smith/isdn4es.htm for some tips on getting more debug info. At the bottom of the page you find a more uptodate page and some logfiles.. It's not a complete story, just making some notes as I go along.
Hope this helps a bit, if nothing else you can show what you get and who knows, someone will spot the problem?
The most recent 'feature' I'm working on is that the ntpd throws a tantrum, but only when connecting through a modem/isdn link. Connecting through an external router works fine.
I don't think ES is broke, I suspect more BT is doing something funny somewhere.
By the way, connecting to BT with a smoothwall box in between the ES and BT works fine, (Smoothwall or any single floppy router)
Hope this gets you started..
kees