Oppo
Turn Wi-Fi Calling On or Off auf Oppo
Weak indoor or rural cellular reception is the most common reason calls fail or drop on an Oppo phone. Wi-Fi Calling solves this by sending voice traffic over your connected WiFi network instead of the mobile antenna, so call quality depends on your internet connection rather than tower distance.
The toggle sits inside the mobile network settings because it is tied to a specific SIM card, which matters on dual-SIM devices. To turn it on or off, go to Settings ➔ SIM card & mobile data ➔ pick the SIM you want to configure ➔ Wi-Fi Calling, then enable or disable the switch.
When this fixes the problem
Wi-Fi Calling helps when you have a stable WiFi connection but poor mobile signal, for example at home with thick walls or in a building with a shielded structure. It does not help if your WiFi itself is unstable, since call quality will suffer the same way.
When to look elsewhere
If the option is missing entirely from the menu, your network operator has not activated the feature for your SIM or plan. In that case, no setting on the phone will make it appear, and you need to contact your carrier first.