As with the Start, Browser and Messages app, the default app for Phone can be changed by installing any app designed for making phone calls.

Note: If a display lock is set up, alternative phone apps may not be able to run correctly.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Apps
Step 3:  Open the  menu
Step 4:  Tap on  Default apps
Step 5:  Tap on  Phone app
Step 6:  Choose desired  Phone app
  1. Open the Settings
  2. Tap on Apps
  3. Open the menu
  4. Tap on Default apps
  5. Tap on Phone app
  6. Choose desired Phone app

Samsung Instructions