 Internet Engineering Task Force (IETF) draft status. Investigate these issues in all countries and areas of the world where your network is considering a SIP trunk for PSTN access because the capabilities and regulations vary significantly. Supplementary Services Cisco Unified Communications deployments offer a rich set of supplementary services. With the use of SIP trunks, how these services operate might change, and you need to evaluate how they can be maintained when a SIP trunk brings external calls into your enterprise. Following are different areas of supplementary services to evaluate: Voice calls Voice mail Transcoding Mobility Voice Calls Telephony features such as call hold/resume, call transfer, call waiting, three-way confer- encing, distinctive alerting, calling line identification (CLID), calling name, and call toggle can be provided by CUCM Express and CUCM for IP phones and by a voice gateway for analog phones. IP Centrex or Class 5 type features (for example, call forwarding, call screening, call park, call return, and SO on) can be provided by central SIP servers resi- dent in the service provider's network. An analog phone in the enterprise can trigger these features with access codes (typically starting with an asterisk) provided by the serv- ice provider. Cisco Unified Voice Gateways send the access codes in the SIP INVITE message over the SIP trunk to the service provider to trigger these features. Chapter 7: Design and Implementation Considerations 107 Voice Mail You can provide voice mail within the enterprise network in a distributed design using Cisco Unity Express or with a centralized design using Cisco Unity or Cisco Unity Connection. You can also choose to get voice mail services from the service provider using a hosted solution (a cloud-based service). Message Waiting Indicator (MWI) is a visual light on IP phones; it is indicated by a stut- ter dial tone on analog phones. MWI for enterprise-provided voice mail systems is not impacted by SIP trunking, but if a service provider host