Samsung
Set Default Dialer App auf Samsung
On Samsung phones, the Phone app works like any other default app, similar to Home, Browser or Messages. You can replace it with any calling app installed from the Play Store, as long as that app is built to handle phone calls.
To switch the default dialer, go to Tap on Settings ➔ Tap on Apps ➔ Tap on Choose default apps ➔ Tap on Phone app ➔ Set the Default phone app. Once set, the new app takes over incoming and outgoing calls, along with your call log and dial pad.
When this can go wrong
If your phone has a screen lock active, some alternative dialer apps cannot run properly while the device is locked. Calls may fail to connect or the app may not launch at all in that state.
If that happens, either disable the screen lock temporarily to confirm the app works, or check the third-party app's own permissions for lock-screen access. If the issue continues, switching back to the built-in Phone app through the same menu path is the more reliable fix.