/WAN link to reach the head- quarters. This is similar to our centralized trunking solution. 264 SIP Trunking Company P used 3 million minutes a month, costing millions of dollars for PSTN phone bills. The new SIP trunking solution provides a lower cost routing solution for Company Meeting Center audio participants over the internal IP Telephony infrastructure. They have saved a huge amount of money per month having this enabled over Internet Protocol (IP). Figure 12-1 shows the PSTN trunk network connection topology between Company P and Company Q. San Jose, CA Multiple Multiple Conferencing Voice Gateways Voice Gateways Server Company Q Company P (Service Provider) (Large Enterprise) Europe Singapore Japan Figure 12-1 PSTN Trunk Connectivity The conferencing servers are hosted in company Q's data center located in a different city. To process a meeting/conference, the service provider uses an Interactive Voice Response (IVR) system to authenticate the dialer and route the call to the appropriate meeting. Moreover, any employee can schedule a meeting and invite other people to join that meeting. So, when the company has 10,000 to 50,000 employees, it should be well designed to support many active users. When the enterprise migrated from Time Division Multiplexing (TDM) trunking to SIP trunking, its voice network became end-to-end IP-enabled. The calls are originated from IP phones, travel through the Cisco Unified Border Element (CUBE), and then connect to the conference server sitting in the service provider's cloud. The enterprise was using H.323 trunking to connect their Cisco Unified Communications Manager (CUCM) clus- ters and campuses. It uses a gatekeeper to make the proper routing of the calls in the H.323 network. It has not changed their existing H.323 network as part of this migration process. It changed only the PSTN part of the network. So, within the call flow, you see some parts are in H.323 and some parts are in SIP. Figure 12-2 shows the complete net- work diagram with SIP trunk connectivity bet