using System;
using System.DirectoryServices.AccountManagement;

namespace new_ad_exmaple
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                PrincipalContext  AD     = new PrincipalContext(ContextType.Domain, "leeds-art.ac.uk");
                UserPrincipal     u      = new UserPrincipal(AD);
                PrincipalSearcher search = new PrincipalSearcher(u);
                
                foreach(UserPrincipal result in search.FindAll())
                    if (result.VoiceTelephoneNumber != null)
                        Console.WriteLine("{0,30} {1} ", result.DisplayName, result.VoiceTelephoneNumber);

                search.Dispose();              
            }

            catch (Exception e)
            {
                Console.WriteLine("Error: " + e.Message);
            }
        }
    }
}