ften routed to a central voice gateway at a campus location and then to the branch office via the enterprise's IP network. Chapter 6: SIP Trunking Models 83 Emergency calls from the site are routed using the local PSTN voice gateway to the local authorities. Small sites use low-density analog or Basic Rate Interface (BRI) trunks. These often become noncost-effective above 8 channels to 12 channels, at which point digital T1 or E1 connectivity becomes cheaper even if the full 24 or 30 channels are not needed by the site. This situation results in sometimes significant PSTN oversub- scription in the network as a whole. Choosing a SIP Trunking Model Adding SIP trunk PSTN access to your network might imply changes to both call routing and the placement of SIP trunk access points in the network. SIP trunk calls have fewer geographic dependencies than TDM trunks. When determining where to connect a SIP trunk in your network, consider the following: The types of calls carried by the SIP trunk. Whether the cost of the service offering is based on a single or multiple physical entry point. Whether the service offering provides international call access. What organization in your network controls the physical termination of SP traffic into your network. The following sections describe these considerations in detail. In addition to working through the considerations and designs discussed in this chapter for a single SP, you might want to consider connecting to multiple SPs either for capacity, cost, or redundan- cy reasons. If you connect to multiple SP SIP trunks, the considerations that follow apply to each one of those in the same manner, and you might even choose different models for each. For example you might select SP X with a centralized model to serve your contact center sites or endpoints, whereas you select SP Y with a hybrid model for your normal business call needs. Types of Calls Carried by the SIP Trunk SIP trunk providers often make a distinction between long-distance and local calls. The concept of local ca