The Mitel asked for G722 first:
m=audio 3000 RTP/AVP 9 0 8 101 (Line 32)
The Grandstream asked for G711u.
m=audio 38976 RTP/AVP 0 9 18 2 101
That’s why the Grandstream isn’t working. It’s negotiating G711U, but then trying to use it as G722.
Edit: If you do a ‘core show channel [tab]’ and figure out the channel of the call, asterisk will think it’s G711, no matter what the phone says.
Edit 2: I bet the ‘but it works when I do xyz’ comments end up disabling every other codec so the Grandstream has no choice BUT to negotiate G722 correctly. That’s why it sorta kinda works, sometimes.