different SIP Trunk service for call centers than for standard DID calls. The difference normally stems from the distinct calling pattern and Chapter 3: Transitioning to SIP Trunks 39 features of call centers (that is, one number with thousands of incoming calls of varying duration). Inbound call centers do not normally make outbound calls (except for trans- fers); however, they do need the capability to burst above predefined call limits. (For example, if 200 trunks were purchase, inbound call centers normally want the capability to accept the 201st call if possible.) Transitioning inbound call centers from TDM to SIP trunks, can result in: Additional flexibility in deployments because the call center can be outsourced by changing where the SIP messages route. A high degree of cost-savings because incoming call centers often generate a large number of minutes and can be responsible for a high percentage of a company's TDM carrier charges. Improved resiliency because call center overflows and outages can be handled quickly and easily by rerouting the SIP messages. Companies are also focused on merging their incoming and outgoing call centers, and as such, a consolidated SIP trunking architecture for both can result in additional cost-saving. In summary, transitioning call center traffic is often an effective early transition option for companies looking to deploy SIP trunks. Outbound call centers are a good candidate for early transition to SIP trunking because of the low complexity of the solution. The risk of misrouted calls is low, and few issues with nonvoice calls occur. Transitioning inbound call centers from TDM to SIP trunks, even with the issues of possible misrouted calls and the additional complexities in possibly having to handle fax or other nonvoice (such as Baudot connections) calls, can result in some of the largest cost-saving when transitioning to SIP trunks for PSTN access. Phase VII: Transition to SIP Trunking at Headquarters Locations The capacity requirements at large headquarters locations 