I have about 20 users on Callto365, similar integration as Call 2 teams. The integration is via MBG, and each user will be configured as a generic SIP phone. The "Call to" service is a cloud based man in the middle, handling the call between Teams and Mitel. It actually works quite well. Call to 365 also has a trunk option, which I am curious about configuring as a SIP trunk to the Mitel PBX, via MBG, in hopes that I can use the Mitel as a trunk gateway to access local PSTN. You will require subscription to E3 and phone system license add-ons or E5. I just recently enabled call queues and was able to leverage call to 365 to route inbound PSTN calls into a Teams call queue as well as a Teams attendant. Pretty slick.