We use the Netvanta 3140 and Adtran TA908's... A LOT... we are not only a Mitel dealer, but a hosted SIP service provider as well and the Adtran is our go to for SIP trunking SBC.
I can possibly give you some sample setup configs, but here is a real general idea... You need a public IP address direct to the Adtran with no firewall or router ahead of it, and a LAN IP on the voice VLAN, both static and dedicated to the Adtran. Then you make two trunk groups, one for the connection to your SIP service provider and one to the Mitel, then in your digit acceptance tables you put the DID's so they are allowed with no cost to the PBX trunk group and all others go to the SIP provider trunk group. This is a VERY simplified overview. We never use the GUI except to load a configuration file and diagnostics, or make a quick tweak on a unit in service already. We basically have two templates, one for "dumb" PBX's which don't handle PAI, BTN, and redirect headers properly (most phone systems, including the MiVO 250), and "smart" PBX's which do handle them correctly like the 3300 and most Asterisk boxes.
Oh yeah, and make sure you have a Netvanta 3140 with a SBC session license of sufficient size, they come in 5, 10, 25, 50, and larger increments. Without the SBC Session license you will never get anywhere. You need one that is a part number like 4700341F2#10 which is a NetVanta 3140 10 SESSION SBC unit.
And you don't need an MBG, but if you have one you most likely don't need the Netvanta either as the MBG handles everything for you.