Using the RingCentral C-Sharp SDK you can download the binary content as shown below:
RestClient rc = new RestClient("ClientID", "ClientSecret", false);
await rc.Authorize("username", "extensionNumber", "password");
...
var extension = rc.Restapi().Account().Extension();
var messages = response.records;
// fax
var message = messages.Where(m => m.type == "Fax" && m.messageStatus != "SendingFailed" && m.attachments != null && m.attachments.Length > 0).Skip(3).First();
var content = await extension.MessageStore(message.id).Content(message.attachments[0].id).Get();
System.IO.File.WriteAllBytes("filename.ext", content.data);
See detailed sample code from here