oying SIP trunks in your network-pulling together for practical use much of the material discussed in more detail in preceding chapters on SP evaluation and network design considerations. It guides you through the stages of planning, evaluating a SIP trunk offering, setting up and running a Pilot Trial, all the way to deploying production service in your network. The last section of the chapter provides a summary of best practices when implementing a SIP trunk, including practices in the areas of choosing an SP, general deployment, network design, choosing protocols and codecs, CUCM and SBC-specific practices, security, and redundancy. Chapter 12 Case Studies This chapter covers the following topics: An enterprise connecting to a service provider Distributed SIP trunking to connect PSTN This chapter examines two case studies of real-life Session Initiation Protocol (SIP) trunk- ing deployment scenarios. Enterprise connecting to a Service Provider (SP) Distributed SIP trunking to connect Public Switched Telephone Network (PSTN) You learn about the different challenges customers face to make a successful deploy- ment. To maintain the confidentiality of the customer network, we cannot mention the actual names of the customers and their networks, but we try to sketch solutions where possible. Enterprise Connecting to a Service Provider In this case study, we show you how a large enterprise (Company P) tried to interconnect its unified communications network through SIP trunking with an SP (Company Q) that provided a conferencing solution for Company P. We also try to explain the complexity and the overall solution of the network. Initially, Company P's internal conferencing calls were routed over PSTN to Company Q's media gateway. Company P has operations all over the world. When any user from Company P dialed from any of Company P's locations to Company Q, that call came to a centralized location (say, headquarters), and then was terminated to Company Q over the PSTN network. Internal employees used an internal MPLS