The default phone app can be changed on the OnePlus smartphone at any time if additional functions or the design of the layout leaves something to be desired.

Additional phone & dialer apps can be downloaded and installed from the Google Play Store.

Android 10 | OxygenOS 10.0
Default Dialer-App -  Open the  Settings
Default Dialer-App -  Tap on  Apps & notifications
Default Dialer-App -  Tap on  Default apps
Default Dialer-App -  Tap on  Phone app
Default Dialer-App -  Choose the  default Phone app
  1. Open the Settings
  2. Tap on Apps & notifications
  3. Tap on Default apps
  4. Tap on Phone app
  5. Choose the default Phone app

OnePlus Instructions