IntelliSMS objIntelliSMS = new IntelliSMS();

            objIntelliSMS.Username = "username";
            objIntelliSMS.Password = "password";

            String MobileNo=(Session["mobile"].ToString());

            String Message = ("Thank you " + Session["full_name"].ToString() + ".Your taxi has been accepted by " + Session["company_name"].ToString() + ". If you want to cancel then please contact the taxi company on " + Session["phone"].ToString() + "."); 
            String Companyname = ("Just-Taxis.co.uk");

             objIntelliSMS.SendMsg (MobileNo,Message,Companyname);

System.NullReferenceException: Object reference not set to an instance of an object.