I'm using Voip.ms with Linphone and everything works fine when the app is open. Calls ring and SMS is recieved. When I close the app or the phone is in sleep mode, calls do not ring at all. It all works normally and rings in the background if I change the transport mode to TCP (and change the voip.ms port so match). This happens on mobile data as well as WiFi. I am using Mullvad VPN as well.