For voice calls from Office Communications Server 2007 to reach the Public Switched Telephone Network (PSTN), a third-party media gateway is required to bridge the IP network and the PSTN network by translating the signaling and media to the protocols used by each network, as shown in Table 3-2.
Table 3-2. Protocol Translation Performed by Media Gateway
SS7 and others
RTP (Real-Time Protocol)/SRTP (Secure RTP)
Such a media gateway is provided by Microsoft partners. (See the Additional Resources section at the end of this chapter for the link.) ...