ent of a SP call agent. Table 5-1 Mapping of IP Addresses to FQDN Fully Qualified Domain Name Phone Number CustomerA.customer.com +1-408-555-1212 CustomerB.customer.com +011-44-555 CustomerC.SecondRange.edc 1.1.1.1 408-555-1212 The SP call agent normally has access to the customer database that holds essential infor- mation that a SP has determined about each customer. Some of this information is not essential for ensuring that the call is complete, such as billing addresses, but other infor- mation, such as the maximum number of calls that the customer has paid to accept, might be an important parameter required to determine the accurate routing and disposi- tion of a call. Some call agents might also function as the routing node for emergency services such as 911 calls. In these cases, supplementary information such as the physical location stored in the customer database might be required to ensure that the call is rout- ed to the correct Public Service Answering Point (PSAP). Generally, the SP call agent is the entity that generates the fully defined SIP messages that eventually make it to the enterprise IP-PBX, and it is the entity that interprets response to these messages. The capabilities and design of the call agent that is deployed is the major differential amount SP is offering to similar SIP Trunking services. The features available in the call agent are usually what determine the final feature set that the SP offers; however, some supplementary services can be accomplished by exter- nal servers. Architecturally, a call agent is most often a back-to-back user agent (B2BUA) in SIP termi- nology, and the general architecture and important interconnects of a SP call agent are shown in Figure 5-2. Chapter 5: Components of SIP Trunks SP Call Agent Edge SBC Terminates INVITE Determines routing for INVITES and how to route based on the dialed number of calls Edge SBC Has access to customer information Media Gateway database to determine, and may use that to assist in routing calls Acts as a Back to Back User