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 to firstname.lastname@example.org), 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!
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Team messaging, video meetings
and phone - all in one app.
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