So a bit of an update.
Moved to custom, private ip on the ICP side, public ip on the Set side. Still same problem. Reviewed the Twilio integration Mitel Doc and started changing settings one by one. Discovered that Twilio does not require "SIP Registrar FQDN or IP Address" in the config. Removed this and now some progress. The far end is ringing (my cell phone), but when I pick up the call on my cell, it continues to ring on the 6930 phone. Appears the "Hey, the far end picked up" message is not getting back to the MBG and/or MiVB.
Fun times.