We have one client that uses our SIP trunks with a BT Versatility, so it does work with a few conditions.
You will need the broadband plus module on the Versatility, with a later version of the firmware to allow you to configure it.
I remember it was a little bit of fun getting it configured to start with, but it now works well. If it's of any help, if you are able to get the the config on one of your PCs I'd be happy to get one of the guys to connect in and set it up to ours to test it works?
BT don't support it, so they won't help if there is an issue, but I'm guessing your our of support anyway?