"Sending SMS from/to extension numbers is not available"
What is the issue?
You can open a case support and ask the backend support team to detect the actual reason for the issue from log.
You can then take appropriate measures to counter that with steps mentioned above
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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