For our new sandbox account we are getting Reg MSG-242", "message" : "The requested feature is not available".. can you please let us know the reason for this code? In my account it's showing active.
Can you check if you are able to send messages via softphone.
Same issue I can find here
Also check the phone number you are sending the sms request in api
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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