Guys,
We ended up using a TelcoBridges ProSBC for this as Mitel has no way to make this happen natively.
We put the ProSBC in the Cloud and then point our MCD to that via an MBG, because we have an MiVCR doing SIP Trunk recording, and then let the ProSBC determine the routing based on call-type.
The ProSBC will see the 410-Gone as an Invalid response and then recursively sends it to the second option.
If anyone has to do this make sure to turn off Use Session Timers as that will cause some troubles with calls dropping and it isn't needed for this solution as you just want calls to pass through it once they are setup.
ProSBC Webportal -> SIP -> Sessions Timers -> uncheck “Use session timer” , save and activate the configuration.
Thanks,
TE
neat. thanks for sharing an update.