I am facing error in SMS API integration....

The method or operation is not implemented.

throw new NotImplementedException();

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



namespace WebApplication31
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        string MyUsername = "923228279278"; //Your Username At Sendpk.com 
        string MyPassword = "4379"; //Your Password At Sendpk.com 
        string toNumber = "923228279278"; //Recepient cell phone number with country code 
        string Masking = "SMS Alert"; //Your Company Brand Name 
        string MessageText = "SMS Sent using .Net";
        string jsonResponse = SendSMS ("Masking", "toNumber", "MessageText"," MyUsername", "MyPassword");

        private static string SendSMS(string p1, string p2, string p3, string p4, string p5)
        {
            throw new NotImplementedException();
        }
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            String URI = "http://Sendpk.com" +
            "/api/sms.php?" +
            "username=" + MyUsername +
            "&password=" + MyPassword +
            "&sender=" + Masking +
            "&mobile=" + toNumber +
            "&message=" + Uri.UnescapeDataString(MessageText); // Visual Studio 10-15 
            //"&message=" + System.Net.WebUtility.UrlEncode(MessageText);// Visual Studio 12 
            try
            {
                WebRequest req = WebRequest.Create(URI);
                WebResponse resp = req.GetResponse();
                var sr = new System.IO.StreamReader(resp.GetResponseStream());
                //return sr.ReadToEnd().Trim(); 
            }
            catch (WebException ex)
            {
                var httpWebResponse = ex.Response as HttpWebResponse;
                if (httpWebResponse != null)
                {
                    switch (httpWebResponse.StatusCode)
                    {
                        case HttpStatusCode.NotFound:
                        //return "404:URL not found :" + URI; 
                        //break; 
                        case HttpStatusCode.BadRequest:
                        //return "400:Bad Request"; 
                        //break; 
                        default:
                            break;
                       //return httpWebResponse.StatusCode.ToString(); 

                    }
                }
                //return null; 
            }
        }
    }
}