There seems to be an issue with your sandbox API.
I am getting for almost any request a "504 Gateway Time-out".
Only a few request are being processed.
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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