If you want a better chance for help here on the developer community, then put a few lines of codes (without any sensitive information such as a real phone number etc.) around the area that causes the problem in your question. Theoretical question requires an entire book on error handling, while some exact code probably requires a few lines of suggested code to fix.
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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