Due to a limitation that is coming from OxygenOS another step is needed to get the Nebry UC app working.

1. After you install the Nebry UC app, proceed with closing the app from any background process.

2. Download another application from the app store, named 'Shortcut Maker'.

3. Once 'Shortcut Maker' is installed, open it, and go to 'Activities'

   

4. Search for 'Calling Account', and under 'Call Management' select 'com.android.server.telecom.settings.EnableAccountPreferenceActivity'

   

5. Click on 'Try this Shortcut'.

   

6. Enable 'Nebry UC' as calling account.

   

7. Close everything, and then open the Nebry UC app again, the calls should now start working.