Alternative telephone apps can be installed and used on the Oppo smartphone for making calls, but to ensure that the right app is called up when a call comes in, it must be defined as the default app.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  App management
Step 3:  Tap on  Default app
Step 4:  Tap on  Phone app
Step 5:  Tap on  App  you want
  1. Open the Settings
  2. Tap on App management
  3. Tap on Default app
  4. Tap on Phone app
  5. Tap on App you want

Oppo Instructions