Well, done a bunch of testing with this and Linphone's mobile softphone seems to work on Android quite well, but I can't get their desktop client to even register...
Bria Solo works pretty good if you disable the Push Server on mobile, and their desktop client works really well. Might recommend that route as this user is an owner of the business and will be living on the road for about a year out of an RV and will not be easy to troubleshoot if they have issues, but Bria holds the configuration on their website rather than directly in the client (except mobile specific settings, like Push Server)... and they want to be a backup for the receptionist (as much as I have recommended NOT doing this).
They also want to see about getting a 5340 setup in the RV on Starlink... my company owns one (Starlink) but I have yet to test the feasibility of that. Weather is nice today, if the CTO is in the office today I might try to set it up and test it.
Would be really nice if I could figure out how to make the Mitel deal with Bria's Push Notification server, it would really help on battery, but every time I turn it on an incoming call to the mobile client fails. It registers the Push Notification server as a "user" while the phone is idle/screen off and then tries to register it on an incoming call to answer, but the Mitel really doesn't like that setup. Will probably start a new post about this and see if anyone has been able to make Push work with the MiVO 250.