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As per this link: https://community.ringcentral.com/questions/1293/meetings-in-sandbox.html

"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!"

Also, you can check: https://community.ringcentral.com/questions/1183/cannot-create-meetings-in-developer-sandbox-accoun.html

and https://community.ringcentral.com/questions/7925/in-order-to-call-this-api-endpoint-user-needs-to-h-3.html