Trying to retrieve list of rooms using EWS Managed API but it returned empty collection

Hi,

I'm trying to retrieve list of rooms using EWS Managed API. Before I have tried the following code I make sure our Administrator put all the rooms in a list following  link https://technet.microsoft.com/en-us/library/ee633471%28v=exchg.141%29.aspx.

Here is the simple code I have used :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Exchange.WebServices.Data;
using System.Net;
using System.Security;

namespace ExchangeConsole1
{
    class Program
    {
        static void Main(string[] args)
        {
            ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);

            service.Credentials = new NetworkCredential("username", "Password1", "domain");
            service.Url = new Uri("https://email.xxxx.com/ews/exchange.asmx");
            EmailMessage email = new EmailMessage(service);


            EmailAddressCollection listOfRoomList = service.GetRoomLists();
            // Display the individual rooms.
            foreach (EmailAddress address in listOfRoomList)
            {
                Console.WriteLine("Email Address: {0}", address.Address);
            }

        }
    }
}

service.GetRoomLists() results "Enumeration yielded no results". Could anybody help me please ?

Thanks

Sandipan