hich gateway to use depends on many factors, including their availability, remain- ing call capacity, and cost for terminating to a particular number. For the proxy to do this adequately, it needs to have access to this information in real-time. This means there must be some communication between the proxy and the gateways to convey this information. In TRIP, the routing tables are exchanged when they change updates sent and exchange the open message and send the 'keep alive' message frequently between peers. After connection, the LS exchange their entire routing table and incremental update messages are sent as the routing table changes. When synchronized, the TRIP routing table of all internal peers are identical. The routes external or internal or local are stored in the Telephony Routing Infor- mation Base (TRIB). The LS collect information and use it to reply to queries about routes to destinations. Any directory access protocol can be used, for example LDAP or DNS. 21.3.4 Implementing Intelligent Network Services in VoIP Application The SELECT_ROUTE PIC allows selection of the pass route to the next server. The route can be according to the INVITE request, a URI or E.164 number. The ENUM standard described an architecture based on the DNS system corresponding to an E.164 number into the communication services identifier, with an order of priority (e-mail, URL of web site, address SIP server, etc). The ENUM protocol therefore finds the various addresses of the user from a simple telephone number. The user can also personalize who can join, with just an E.164 number. It is easy to add or to modify this supplemen- tary information without changing the number used for the access. Thereby, the protocol is considered as a technical bridge of correspondence between the Internet and the PSTN, thus allowing the interoperability between these two systems [10]. DNS stores information in different types of records. The NAPTR record is used for identifying available ways of contacting a specific node identified by that 