Watching an advertisement is one option to earn Credit balance. Alternatively, a user can pay through Apple account to purchase a Credit pack of choice - all within the app.

The subscription ensures that the number stays with you as long as you are subscribed to an uninterrupted plan. Whereas the Credit balance entitles you to make outbound calls/Texts (till you have a balance left).

A user may need to use the app for inbound calls/Texts only, for such, 0 Credit balance still works! Such users only require an active subscription.