Get SIP address from active directory

The below code will be used to get the SIP address from the active directory. SIP address is commonly used in office communicator server. I have used this code to fetch the SIP address. Based on the SIP address, we have applied provisioning on office communicator server 2007.

 private string GetSIPAddress(string email)
        {
            DirectoryEntry directoryEntry =
                     new DirectoryEntry(ConfigurationManager.AppSettings["LDAPPATH"]);

            string resultValue = "";

            using (HostingEnvironment.Impersonate())
            {
                DirectorySearcher directorySearcher = new DirectorySearcher(directoryEntry);
                directorySearcher.Filter = string.Format("(&(objectClass=user)(objectCategory=user) (mail={0}))", email);

                directorySearcher.PropertiesToLoad.Add("msRTCSIP-PrimaryUserAddress");

                SearchResult result = directorySearcher.FindOne();

                if (result != null)
                {
                    if (result.Properties["msRTCSIP-PrimaryUserAddress"] != null)
                    {
                        resultValue = result.Properties["msRTCSIP-PrimaryUserAddress"][0].ToString();
                        resultValue = "'" + resultValue + "'";
                    }
                }
            };
            return resultValue;
        }