Here's one of our developers with a question on how to retrieve accountId and extensionId from your RingCentral account.
"Parameter accountId is required in most of the API resources, may I know what is the account Id in sandbox account? Or where is it found?"
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The RingCentral service allows its customers to create and register an account that is usually associated with the customer's company. After registering the account with the company main number the user can create extensions of different types and functionality. The extensions can further be assigned with the phone numbers and phone devices.
All the account related link could be looked up in the "Exploring Account and Extension settings" section of our Developers guide here
https://developer.ringcentral.com/api-docs/latest/index.html#!#ExploringAccount.html