SMS API "The requested feature is not available" in sandbox

I created a new sandbox account for testing the SMS API. I then created a console app (per ringcentral instructions) and tested the API using the sand box credentials. Connecting to the API works fine but returns this error when trying to send a test SMS:

ringcentral.http.api_exception.ApiException: The requested feature is not available

I have made sure I've added the correct features to the sandbox app (per the instructions).

Thanks in advance.