Here code for IService1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.Text;

namespace WcfService
{
    [ServiceContract(SessionMode = SessionMode.Required, CallbackContract = typeof(IService1Callback))]
    public interface IService1
    {
        [OperationContract(IsOneWay = true)]
        void AddNumber(int number);
    }
    public interface IService1Callback
    {
        [OperationContract(IsOneWay = true)]
        void Calculate(int result);
    }
}

Here codes for Service1.svc.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.Text;

namespace WcfService
{
     [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
    public class Service1 : IService1
    {
        public void AddNumber(int number)
        {
            callback.Calculate(number + 10);
        }
        IService1Callback callback
        {
            get { return OperationContext.Current.GetCallbackChannel(); }
        }
    }
}

Web.config မှာ ထပ်ပြီး system.serviceModel => services=> endpoint ရဲ့ binding method ကို wsDualHttpBinding လို့ ပြင်ပေးရပါမယ် .. ဒါမှ Duplex Callback method က အလုပ်လုပ်မှာ ဖြစ်ပါတယ် ..
Codes for Form1.cs
Form1 have only 1 button ..

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.ServiceModel;
using System.ServiceModel.Channels;

namespace Client
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            InstanceContext instanceContext = new InstanceContext(new MyCallback());
            ServiceReference1.Service1Client svcClient = new Client.ServiceReference1.Service1Client(instanceContext);
            svcClient.AddNumber(33);           
        }
        public class MyCallback : ServiceReference1.IService1Callback
        {
            public void Calculate(int result)
            {
                MessageBox.Show("result=" + result.ToString());
            }
        } 
    }
}

Here some useful links ...
http://www.c-sharpcorner.com/UploadFile/dhananjaycoder/3189/

http://netprogramminghelp.com/wcf/how-to-createperform-duplex-service-contracts-in-wcf/

http://www.codeguru.com/csharp/.net/net_wcf/article.php/c17831__1/Creating-a-Duplex-Service-in-Windows-Communication-Foundation-WCF.htm

MSDN link