Phone calls are possible with both physical SIM cards and WiFi calling. But physical SIM card requires a SIM card call, but the WiFi calling system does not require any SIM card. This is mainly done through a WiFi connection.
Oppo Reno8 (China) phone offers dual SIM and WiFi calling. Like other Wi-Fi calling supported phones, you can communicate with your acquaintances using only a Wi-Fi connection. But before that, you must enable the WiFi calling system from phone settings, and you don’t need to install any app.