From the screenshot above, you are trying to pass the parameters as body with a GET request which does not seem correct.
If you check the API reference here, the parameters are actually query params.
Could you make the changes and try again?
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.