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After sending a fax, you can detect the message status and the message id from the response. The status is normally in "Queued" as you already saw. You can use the message id and call this API to detect the latest status. E.g.

async function read_message(messageId){
    var resp = await platform.get('/restapi/v1.0/account/~/extension/~/message-store/' + messageId)
    var jsonObj = await resp.json()
    console.log(jsonObj)
}