Add a static route to amc-sync.mitel.com (216.191.234.91) to use your LAN gateway... problem solved. We have a few locations that do this.
It is not ideal, but if you have other means of accessing the network and are only using it as a SIP gateway you should be fine.
Does the SIP hosting company provide an Adtran Netvanta or something similar for it's trunks, or is it an ISP handing off directly on a modem/ONT interface? If they are handing you off via a SIP gateway, ask them to change the address on the hand-off interface to one that is on your voice VLAN and ingress it directly, we do this for every install for 99% of our SIP trunking customers.
Honestly, it might be easier and more reliable to just get a hardware solution like a SIPerator or Adtran Netvanta 3140