In order to successfully make a RingOut call please use main company number * extension in the from section OR the custom number section.You can add the digital line as the caller id as shown in screenshot.
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
6 People are following this question.