As per this link it is not available in sandbox:
There is no way to create meetings in sandbox.
Upon developer's request (send account's info through this support ticket to the developer support team), we can enable the meeting feature in the sandbox so developer can call the meeting APIs. However, the meeting information such as startUri and the joinUri etc. are just dummy info and cannot be used to launch the meeting app.
Real meeting can only be tested on the production environment!
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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