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That is exactly what we have used to create our fax request. Is there a specific format for the phone or "to" values as the examples don't show how those are formatted. We use the .Net SDK to create all of this, yet it does not work.

I can't find this object name "CreateMessageRequest" from the error trace you posted so I don't know where did you see it. Did you create the request using that object?

I just sent a test fax again using my code and it worked well.

/*
Send a high resolution fax message to a recipient number
*/
static private async Task send_fax()
{
    var bodyParams = new CreateFaxMessageRequest();
    bodyParams.to = new FaxReceiver[]
          new FaxReceiver { phoneNumber = "+1209252XXXX" }
    };
    bodyParams.faxResolution = "High";
    bodyParams.coverPageText = "This is a demo Fax page from C#";

    var attachment = new Attachment { filename = "test.jpg", contentType = "image/jpeg", content = System.IO.File.ReadAllBytes("test.jpg") };
    var attachments = new Attachment[] { attachment };
    bodyParams.attachments = attachments;

    var resp = await restClient.Restapi().Account().Extension().Fax().Post(bodyParams);
    Console.WriteLine("Fax sent. Message id: " + resp.id);
    await check_fax_message_status(resp.id.ToString());
}

/*
Check the sending message status until it's no longer in the queued status
*/
static private async Task check_fax_message_status(string messageId)
{
    try
    {
        var resp = await restClient.Restapi().Account().Extension().MessageStore(messageId).Get();
        Console.WriteLine("Message status: " + resp.messageStatus);
        if (resp.messageStatus == "Queued")
        {
            Thread.Sleep(10000);
            await check_fax_message_status(resp.id.ToString());
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
}

If everything is correct in your code, maybe it's truly the to fax number is invalid. You can submit a dev support ticket to get help.

Ok, let me stub your test code into our API and see if it sends. You don't see the Type CreateMessageRequest because this is an HTTP response, so it doesn't know about the type. But when building the request to send, we are using the CreateMessageRequest from the RingCentral SDK.