Hi there. You are currently using our legacy fax API which was recently upgraded to require TLS 1.2 for security purposes. The error you are receiving indicates that your system does not support TLS 1.2. Please see our Knowledge Base article 8778 on how to resolve this:

TLS 1.2 KB Article: https://support.ringcentral.com/article/8778.html

Additionally, you may want to consider upgrading to our new API which you can learn about at https://developers.ringcentral.com.

We also have a C# SDK available for the new API:

C# SDK on GitHub: https://github.com/ringcentral/ringcentral-csharp

C# SDK on NuGet: http://www.nuget.org/packages/RingCentralSDK

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I am using the below code. Can you help me what should do change here?

Dictionary<string, object> postParameters = new Dictionary<string, object>();            postParameters.Add("Username", ConfigurationManager.AppSettings["FaxAPIUserName"]);
            postParameters.Add("Password", ConfigurationManager.AppSettings["FaxAPIPassword"]);
            postParameters.Add("Recipient", "1"+BillFax+"");
            postParameters.Add("Coverpage", "0");
            postParameters.Add("Coverpagetext", "JLC Invoice");
            postParameters.Add("Resolution", ConfigurationManager.AppSettings["FaxResolution"]);
            postParameters.Add("Attachment", new FormUpload.FileParameter(data, "invoice.pdf", "application/pdf"));

            // Create request and receive response
            string postURL = ConfigurationManager.AppSettings["FaxAPIUrl"];
            string userAgent = "JLC";
            HttpWebResponse webResponse = FormUpload.MultipartFormDataPost(postURL, userAgent, postParameters);


Please help me regarding this. 

I tried the c# sdk but it cannot support my project. It raised some exception.

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