It seems my problem is exactly the same as Daniel Garner's. iOS 15.6, Linphone 4.6.5
I've tried all of the following, but nothing has worked:
This is an unlocked iPhone SE. I've reset the phone, uninstalled and reinstalled Linphone many times. I checked Background Apps, and Linphone is there and enabled. I've cleared the network connections (as recommended by a few websites I found). Further, Linphone will not even ring the telephone even when it is in the foreground. It only vibrates. Push is enabled on Linphone.
A technical bit worth noting--On Android, Linphone installs a service that is constantly visible and keeps the phone in contact with the Voip service. On iOS, though, Linphone does not do this. I read that iOS has limitations that prevent that solution, but I have no idea why iOS would be designed that way.
Also, there is extremely little help through Google searches which leads me to think this might be a relatively new problem. I can't imagine with over half a million users of Linphone that only Daniel Garner and I have been the only ones to see it.
If I find a resolution I'll reply to this email.