Maybe the server is temporarily down. You can check the issue by submitting a support ticket to the developers support team.
Saying "High Volume SMS", do you use a phone number which is provisioned for High Volume SMS for your account? Remember that High Volume SMS is not supported in the sandbox.
See more about High Volume SMS
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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