Aha, this makes sense, it could be that Linphone was forcing IPv6 (which is strange to do by default) - appreciate the feedback here.
With Linphone, you can add multiple accounts, in your case, I’d add an account for each of your FlyNumbers - this way, all you need to do is choose the already saved account from the main menu on the left.
Glad to hear this is working now and appreciate your patience throughout - let us know if you run into anything else.