Thanks for the tips! I got a little farther, but still no luck. Probably going to throw the phone out in the garbage soon.
A few thoughts on what I'd look to try...
-Try registering the extension to a soft client like xlite to see if it works at all
I downloaded
Telephone.app from the Mac App Store and installed it on my Mac. After trying to set it up, I discovered that the SIP settings I had been using on my Mitel didn't work there, either. Finally I found a
wiki page at voip.ms suggesting I needed to create a "subaccount". I did that and then was able to successfully make outbound calls from Telephone.app.
I went back to the Mitel web gui and updated the SIP user name and password to use the same "subaccount" credentials I had gotten to work with Telephone.app, but the LCD still reports "*NO REG*" and it won't make calls.
-Does voip.ms use UDP/5060 or something else?
UDP/5060
-Does voip.ms want to authenticate based on MAC or source IP?
In my case, no. voip.ms supports those options but I don't have them enabled and didn't use them for my Telephone.app testing.
-Grab a packet capture of the phone booting up and trying to register
Sadly, my home router (Netgear R6250) doesn't have a packet capture feature. I installed Wireshark on my Mac, but it is a work device and the admins have disabled the Internet sharing feature, so I'm not sure how useful it would be to plug the phone in to it.
It is unfortunate that the Mitel web UI is so limited and doesn't support logging or diagnostics. I tried telnetting into it also but it seems to require a different username and password from "admin"/"5312".