d9780 I might be wrong, but I was under the impression that Mysudo was a VoiP provider rather than a typical mobile carrier. I believe that VoiP can be routed over your VPN.
WiFi calling is something different. It requires a SIM (eSIM or physical), and it routes your phone call over internet via your WiFi to another cell tower if you don't have mobile reception available. It is this WiFi calling which can't be routed over VPN, since your SIM needs to be used to authenticate your device to the mobile network.