Oppo
Change Default Dialer App auf Oppo
When you install an alternative calling app on an Oppo phone, ColorOS does not automatically switch call handling to it. Incoming calls, call logs, and call-related notifications still go through whichever app is currently registered as the default phone app.
This is controlled through the app management section of the system settings, not through the calling app itself. Even if the new app is fully installed and set up, it will not receive calls until you assign it as default there.
Where the setting is located
Open Tap on Settings ➔ Tap on App management ➔ Tap on Default apps ➔ Tap on Phone app ➔ Set the Default phone app. Select the calling app you want to use for incoming and outgoing calls.
Once confirmed, all future calls, including notifications and the call log, are handled by that app. The previous default app stops receiving call events, but stays installed and can be switched back the same way at any time.
If calls still open the old app after this change, check whether the new app has all required permissions enabled, since missing permissions can cause ColorOS to fall back to the previous default.