I acquired a bunch of Mitel 6900 series phones, 6920s and 6930s to be exact.
I have installed a version of Freepbx 17 using Asterisk 21.6, and have managed to get them to connect to it, kind of. I currently have two extensions set up, and when I try to call from one to the other it comes up unavailable. The phone calling connects to the pbx and makes the call, but the pbx can’t connect to the one its trying to call.
I figured out the problem, I was using Sip firmware version 5.1, I upgraded to 6.2.0.14 and it works perfectly. Apparently the older Mitel Sip firmware is still trying to register to a proprietary Mitel server.