Instead of using the c-sharp SDK, which is obsolete, I recommend you to use the latest .Net SDK. And follow the example code below to send a fax.
// set the last param to true for production
RestClient rc = new RestClient("clientId", "clientSecret", false);
await rc.Authorize("username", "extensionNum", "password");
if (rc.token.access_token.Length > 0)
{
var requestParams = new CreateFaxMessageRequest();
var attachment = new Attachment { fileName = "test.jpg", contentType = "image/jpeg", bytes = System.IO.File.ReadAllBytes("test.jpg") };
var attachments = new Attachment[] { attachment };
requestParams.attachments = attachments;
requestParams.to = new MessageStoreCallerInfoRequest[] { new MessageStoreCallerInfoRequest { phoneNumber = RECIPIENT } };
requestParams.faxResolution = "High";
requestParams.coverPageText = "This is a demo Fax page from C#";
var resp = await rc.Restapi().Account().Extension().Fax().Post(requestParams);
Console.WriteLine("Fax sent. Message status: " + resp.messageStatus);