 PSTN connectivity is determined. The hybrid model provides most of the operational and cost benefits of centralized trunking, while also providing contained latency for sites that do not have a dedicated SIP trunk, and high availability alternatives for all sites. Which sites in your network warrant a SIP trunk connection is a decision based on one or more of the following considerations: Country and continental boundaries. SP service areas and DID number availability. 88 SIP Trunking Distance (and therefore latency) of routing remote site calls via an intermediate site, especially for local calls to or from the remote site. How many campus or data centers you have and your existing high availability design between these for other enterprise services. Cost or tariffing on the SIP trunk services you are considering. Whether the SIP trunk provider is also your data network (for example, MPLS) provider. To what extent you are able or willing to upgrade your network provisioning and dial- plan routing to accommodate SIP trunking. Considering Trade-Offs with the Centralized and Distributed Models You need to consider many points when deciding how and where to bring SIP trunks into your network and whether a predominantly centralized or distributed model is best for your needs. Work through the considerations discussed in this section before you engage SPs for proposals on their offerings. Use the points discussed to formulate questions to your own network design IT team and to evaluate SP offerings for SIP trunks. SIP trunk service has not reached the maturity of TDM trunk offerings, and the varia- tions in costs, features, connectivity options, and regulations are far from settled. A thor- ough evaluation of SIP trunk offerings is therefore recommended. DID Number Portability There are well-understood rules in TDM PSTN trunking regarding number portability. Do not assume these rules apply to SIP trunking, even if the service is from the same provider. Your existing DID numbers have geographic attributes and are like