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!