I've tried on two phone types - a 7821, and an 8841 - but the result is the same on both; sometimes the BYE message comes from the ITSP straight away, other times it takes 30 seconds (approx) for the VG to receive the BYE from the ITSP. I contacted the ITSP about it, and they're investigating why in some cases they aren't passing it on straight away. My initial thought was maybe I hadn't configured the SIP profile correctly on the VG to give them the external IP in all the correct places.

Both phones are registered to CUCM as SIP devices.

I'll have a look at getting some logs from CUCM!

It also looks pretty good, the BYE from the ITSP is sent to the CUCM by the VG right away, so it seems like the problem is not with the VG.

It would be interesting to see the messages between CUCM and the phone itself.

What kind of a device is it? 
Is it SCCP or SIP?

You could try collecting CUCM logs for a failed call and load them into translatorX, it should show us what went wrong.

If the failed call is with an SCCP device, I would try to see if it work fine with a SIP based device. It would also be easier to troubleshoot with.

Pasha.