es that SPs can offer on their SIP trunks. Regardless of the specific service offered, the enhanced services server needs to interact with the call agent to ensure that the service is correctly invoked and to interact with the billing server to ensure that an accurate and complete detail of the enhanced service that was invoked is maintained. Depending on the complexity of the fea- ture invoked, the enhanced services server might need to invoke other features that involve interaction with the network layer, (such as setting QoS policy), interaction with the security layer, (such as opening specific pinholes in firewalls to allow additional traffic), or interaction with the customer database to ensure correct routing policies are followed. The features and interconnects of a SP enhanced services server are shown in Figure 5-6. Enhanced Services Server Call Agent A Network Interacts with Call Agents to determine if an Enhanced Feature needs to be invoked Depending on the features, the call signaling may be adjusted in real time to enable the feature Enhanced Services often need to interact with other Enhanced Services Call Agent B Network Call Agents usually invoke Enhanced Services Servers in real time by passing Updates bill and billing flags to indicate SIP messaging up to the server, which usage of features. then manipulates and forwards the messages. Customer Database Figure 5-6 Features and Interconnects of a SP Enhanced Services Server SP Network-Peering - Session Border Controllers SPs focused on deploying trunking services know that the number of endpoints accessi- ble with the trunks directly impacts the potential market acceptance of the trunking offering. Generally, when competing with TDM trunks that offer connectivity to 100 per- cent of valid, phone numbers, there is no alternative for a SP that offers SIP trunks for 72 SIP Trunking PSTN access but to also offer connectivity to 100 percent of phone numbers. Of the fea- tures of the PSTN, wide accessibility with a universal routing plan (for example, 