This also actually addressed the original question from my ticket as well. To solve my initial problem, I was masquerading all my traffic from the Adtran by a weird rule in my Microtik box. This rule would send out my SIP request from local .249 as public .212. After I changed the Domain address (that you recommended I do) and it worked for inbound, I wondered what would happen if I disabled that rule in the Microtik. And, ta da! I didn't need it after all, because now the Adtran is sending out those requests with the correct IP address right from the start.
Another thing I found I did not need (at least with our carrier) as I have been testing were these entries:
register range 4053387505 4053387506 auth-name "4053387505" password "XXXXXXXXX"
I don't know why I was told by an "expert" that I callled why he thought I needed these, but I did not. I only needed the main account login credentials.
And speaking of "experts". The main reason I started this project was to find an alternate way to deliver SIP service that we resell to customers with existing T1 interfaces. So, on the word of both the carrier we sell for, and another company that partners with them, we bought this Adtran to test with. And when I ran into difficulties, all of a sudden, these same sources got amnesia about how to configure it with the provider! The conversation would go something like "Oh, yeah! The Adtran 908! Yeah, we installed a BUNCH of those! Great product!" "Great! Do you have any write-ups on how to configure them?" "Uh, noooooooo........" "What about a database for an example?" "Um, uh, no, don't have that either..."
So, you cannot imagine how grateful I am for the help you provided today. Thanks so much! Since I cleared this hurdle, I have been able to test this box on both a Mitel HX controller with PRI, AND a Mitel 3300/MCD with PRI, and they both work great! (The T1 was the easy part of this whole thing!)
Now, my company has a new option to provide to our customers!