"Sorry, the service is temporarily unavailable" Error while trying to create Sandbox account

Can you clear the browser caches and cookies for the site and try it again. Let me know if the problem persists.

I'm also experiencing this same issue. I've attempted using multiple browsers with the same result.

Please share the UID in order to investigate this further, alternatively, you can also submit a case here https://developers.ringcentral.com/support/create-case