Before you can test your integrated app to make or receive calls you need to add a direct phone number to your RingCentral for desktop application (or Softphone).
Learn more how to add direct numbers to configure a RingCentral Digital Line from the online account by clicking here
(When performing step 7 please select RingCentral for Desktop instead of a physical phone. Also you can disregard any steps that relate to shipping since you have not ordered a desk phone)
If you need to select a new number in the sandbox please pick from the available area codes of 732 (NJ) or 585 (NY)