First of all, I masked your company production number in your question.

Having a look at your account, I see the number is the main company number and it has been provisioned for A2P SMS. This means that you cannot send SMS messages from that number using the /sms endpoint. Instead, you have to use the /a2p/batches endpoint. Also, since it is the main company number, the user who is set to handle calls to that number in your IVR settings is the number owner. And in your case, it is you I guess.

Make sure that your app has the A2PSMS permission.

I am also receiving this error with my sandbox account.

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

Can you please clearly outline the steps that must be taken to enable sending SMS. My sandbox number is +14372538950.

Because the sandbox environment does not support A2P SMS at all if that API call was the /a2p/batches endpoint.

I see the number was not enabled for SMS. I enabled it but not sure if it's enough for now. Try again and let me know.