SMS FeatureNotAvailable in Sandbox

Getting a FeatureNotAvailable error in our sandbox app. Happens in Postman and .NET. The sandbox number is "+16506849983".

Request:

{

    "to": [

        {

            "phoneNumber": "*****"

        }

    ],

    "from": {

        "phoneNumber": "+16506849983"

    },

    "text": "Test SMS message"

}

Response:

{

    "errorCode": "FeatureNotAvailable",

    "message": "The requested feature is not available",

    "errors": [

        {

            "errorCode": "MSG-242",

            "message": "The requested feature is not available"

        }

    ]

}