Should I save contacts to device only, or to both device and server in chat app

I am a beginner coder, and I am learning android through making apps.

My current app is a simple chat app, and what I want is this: When the user installs an app and registers, his contacts should compare to registered users on the server. That sqlite database on the device should be populated with users contacts who have also installed the app and registered.

Now my question is this: should I save registered contacts to sqlite database only, or should I save all the contacts on server when the user registers, and save the registered contacts to device's sqlite database? I've heard that Viber saves all user's contacts on server and displays only the registered ones. Is this the good way to do this, is this the preferred way, does it take too many resources?

Thank you in advance.