Welcome to the RingCentral Community

You can check the following:

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);

ref: https://stackoverflow.com/questions/52486470/how-to-view-download-the-sent-fax-using-ringcentral-fax-api-c-sharp

full code reference: https://github.com/ringcentral/ringcentral-csharp-client/blob/master/RingCentral.Test/BinaryTest.cs