ncoming calls from CUCM, enabling it to use its alternative rout- ing logic to place outgoing calls. In the absence of (4), this is the method that indicates to the CUCM that the service provider SIP trunk is down. Summary SIP trunks are becoming an increasingly viable option for enterprises wanting to deploy IP-based PSTN access. This chapter highlighted many of the network design and imple- mentation considerations you should work through while planning or installing a SIP trunk for production purposes in your network. Migrating to SIP trunking is a fundamen- tal network change that should be accompanied by the appropriate level of planning and configuration and can require several phases of deployment. In Chapter 8, another key area of network consideration-interworking and interoperabil- ity-is explored in further detail to round out the discussion of network design consider- ations regarding SIP trunking. Further Reading The following documents and references provide additional information on the topics covered in this chapter. General SIPConnect Forum: Focused on defining SP UNI compliance as a standard to ease interop requirements. http://www.sipforum.org/sipconnect. Cisco IOS and Unified Border Element Documents More information on TLS configuration for the CUBE can be found on Cisco.com. www.cisco.com/go/cube > Configure > Configuration Examples and TechNotes > Unified Border Element SIP TLS Configuration Example. More SIP Normalization examples for the CUBE can be found on Cisco.com. www.cisco.com/go/cube > Configure > Configuration Examples and TechNotes > Unified Border Element (CUBE) Session Initiation Protocol (SIP) Normalization with SIP Profiles Configuration Example. Voice Performance Statistics on Cisco Gateways. www.cisco.com/en/US/docs/ios/123t/12 3t4/feature/guide/gt th.html. 140 SIP Trunking IETF RFCs Transport Layer Security (TLS) RFC-2246. http://www.ietf.org/rfc/rfc2246.txt?number=2246. A Privacy Mechanism for the Session Initiation Protocol (SIP) (RFC-3323). http://www.ietf.org/rfc/rfc33