Please review your number's options to ensure that the line is enabled and that you are requesting for calls to ring in the app. If you are still not receiving calls, then please try force-quitting the app. Wait a few seconds, then re-open the app.
As an alternative, you can choose to forward incoming calls to another number (such as your actual cell phone number). This will be a better alternative when your internet connection is not so great, and your real number still remains protected.