Login your sandbox or production web service. Check which extension is assigned with this company main number (normally it is assigned to the operator). Login your softphone with that extension credentials then you will be able to receive and view SMS messages for that number.
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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