Site2SMS Send SMS: The web remote server returned error (403) forbidden

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using ASPSnippets.SmsAPI;
using System.Net;

namespace Sms_Demo
{
    public partial class SmsDemo : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

       


        protected void btnSend_Click(object sender, EventArgs e)
        {

            SMS.APIType = SMSGateway.Site2SMS;
            SMS.MashapeKey = "<Mashape API Key>";
            SMS.Username = txtNumber.Text.Trim();
            SMS.Password = txtPassword.Text.Trim();

            if (txtRecepientNumber.Text.Trim().IndexOf(",") == -1)
            {
                //Single SMS
                SMS.SendSms(txtRecepientNumber.Text.Trim(), txtMessage.Text.Trim());
            }

            else
            { 
                //Multiple SMS
                List<string> numbers = txtRecepientNumber.Text.Trim().Split(',').ToList();
                SMS.SendSms(numbers, txtMessage.Text.Trim());
            }

        }
    }
}

also include AspSnippets.SmsAPI in solutionexplorer's Reference folder


got error 
 at following line

//Single SMS
                SMS.SendSms(txtRecepientNumber.Text.Trim(), txtMessage.Text.Trim());

the error is 

the web remote server returned error (403) forbidden