Meeting does not work on sandbox account. When Meetings (RCM) is enabled for your sandbox account, it just allows you to create an app and call the APIs to create a meeting etc. But you cannot test (start, join etc) those meetings at all under sandbox environment. If you have a production account, you can then graduate the app and test it on your production environment.

Thanks. Running on a production account isn't a good option right now. I could have sworn that I had this working on my developer account a couple months ago. I wish I remembered what I did to make that happen.

There is no way that a meeting can be tested in sandbox account. We don't have watermark nor limit the trial limit so it is simply not supported.

https://community.ringcentral.com/questions/1293/meetings-in-sandbox.html