Just an update, ended up opening a ticket with Mitel support.
After a day looking at the tug logs they had determined what my issue was. Apparently to fix an earlier issue with the way that MBG dealt with SIP phones, they changed the way that the MGB could deal with SIP setup messages to only one of the possibilities. The Grandstream phones were trying to use one of the other setup formats and MBG wasn't handling it. They did some background stuff to my MBG 5.2.21 blade to make it work for now, and promised a permanent solution in MBG 5.2.23. (We're running through a MAS demo, so I don't know if it works correctly in 6 or not.)
