If you are using the Android version of the CallTiger app, you can select when CallTiger should handle your calls (Always, Never, Only for international calls, Ask me first) in the app Settings. As soon as you have made your choice you can just call your contacts as you would normally do (from your native Android dialer) and CallTiger will handle the call according to your preferences.
If you are using the iPhone version of the CallTiger app, you need to use the dialer in the CallTiger app. Your existing contact list can be accessed from the app as well.