OPPO A17
It's not possible to use Wi-Fi calling
Solution 3 of 4: Your phone's software hasn't been updated to the latest version
It's recommended that you update your phone with the newest software, as the manufacturer continuously corrects errors. It's a good idea to back up the phone memory first. Before you can update the phone software, you need to set up your phone for internet.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press About device.
Press ColorOS version. If a new software version is available, it's displayed. Follow the instructions on the screen to update the phone software.