This error is seen when I am testing the SMS API
403 means forbidden permission which means you are not authorized or allowed. This happens when you get with wrong credentials like client id or client secret or wrong phone number or extension etc.
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:
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