I am unable to send SMS using the sandbox environment for my username - +14707818835

I am unable to send SMS using the sandbox environment - https://platform.devtest.ringcentral.com

using the number mentioned above. The error says :

{ "errorCode": "FeatureNotAvailable", "message": "The requested feature is not available", "errors": [ { "errorCode": "MSG-242", "message": "The requested feature is not available" } ]}

I have also raised a ticket for the same with support but no response yet. Please help here.