If you are receiving a "503 Service Unavailable" error message when attempting to call an API in sandbox, then be advised that there are a number of RingCentral products that are not yet available in sandbox. These products are:
Each of these API products have a similar, but distinct process for gaining access to these APIs in production. Consult the Developer Guide via the links above to learn how to gain the necessary access. If you still need help, please reach out to our support team, who would be happy to guide you.