ll routing in your network for the most cost-efficient calling patterns. This optimization, in turn, can affect current call admission control (CAC) and bandwidth-allocation policies implement- ed in your network. Some specific items that might affect your dial-plan and call-routing configuration include If you currently have a separate dedicated Time Division Multiplexing (TDM) Public Switched Telephone Network (PSTN) voice gateway per Cisco Unified Communications Manager (CUCM) cluster or IP Private Branch Exchange (PBX), then you have a single enterprisewide SIP trunk shared between them. If the SIP trunk offers only long-distance (or certain types of inter-regional) calls, then your TDM PSTN gateways offers both local and long-distance calls. Chapter 7: Design and Implementation Considerations 105 Whether the SIP trunk is going to be used by all the users in your network (all sites) or only by users colocated at the site where the SIP trunk terminates. Whether routing of emergency or fax, modem, Point of Service (POS), or Telecommunications Device for the Deaf (TDD) calls need to be rethought because they might not initially make use of the SIP trunk service. Certain service providers require that a "+" be added to the front of a phone number sent on a SIP trunk. Specifically, the From field in a SIP message must be valid, as in From: +14085551212. When interconnecting through CUCM, this configuration can be accom- plished by using translation rules on a Cisco Unified Border Element (CUBE) between CUCM and the SIP trunk service provider. Certain SIP trunk providers require users to complete a registration before they can use the service. This security practice is a good one for service providers to ensure that calls originate from only well-known endpoints. CUCM does not natively support registra- tion on SIP trunks, but this support can also be accomplished by using a CUBE. The CUBE registers to the service provider with the phone numbers of the enterprise on behalf of CUCM. Two additional considerations reg