The decision about deploying a branch site SIP trunk requires a site-specific cost analysis. For example, an enterprise that has a central site in Redmond, Washington, and a branch site in New York should do an analysis to determine whether to implement a SIP trunk from the New York site to a local service provider.

To determine whether a distributed SIP trunk in New York is cost-effective, identify which DID numbers will use the SIP trunk, and analyze the number of calls New York makes to areas other than Redmond (425). You can have DID termination for the branch site at the central site. For example, the Redmond central site can host DID numbers for the New York branch site. If the cost of implementing a distributed SIP trunk is less than the cost of those calls, consider implementing a SIP trunk at the New York branch site.

The choice between a deploying a SIP trunk instead of a gateway is based on the difference between the PSTN long distance toll charges of each option. If you deploy a branch site SIP trunk, you also need to determine your resiliency and bandwidth requirements. If the link between your branch site and central site is resilient and has sufficient bandwidth, you can deploy a SIP trunk or a gateway. You do not need to deploy a Survivable Branch Appliance at the branch site. If the link between your branch site and central site is resilient, deploy a Survivable Branch Appliance or Survivable Branch Server, and then deploy either a gateway or SIP trunk at the branch site.