1. Dial into and join Teams meeting from desk phone with conference ID.
2. Approve dial-in user from Teams and mute them.
3. Hit Add User button on desk phone.
4. Dial into and join Mitel conference bridge (x88200) with conference ID.
5. Hit Join Calls. 

The public can then dial into the Conference Bridge.  They'll be admitted automatically, but since the connection from Mitel into Teams is muted, the public cannot speak, and they cannot unmute themselves.   

Uses one phone line to dial into Teams, and one phone line for each caller.