arding call routing include Direct Inward Dial (DID) number reachability Emergency call routing Porting Phone Numbers to SIP Trunks When an enterprise starts using a SIP trunk for incoming calls, the phone number must be ported to this service. When external end users call the number, rather than ringing at the traditional TDM gateway owned by the enterprise, it rings in the service provider's core network, and the call is routed to the enterprise with the SIP trunk. Because of the complexity of porting phone numbers, most SIP deployments find it easi- er to start services with outbound calls or with inbound contact center toll-free service calls (non-DID). It is important for the enterprise to understand the timelines and transi- tion plans offered by the service provider for porting DID numbers. Enterprises' business users cannot afford to be unreachable on their primary PSTN phone numbers while this porting activity occurs. Emergency Calls Emergency calling is an important consideration to account for when integrating SIP trunk access into the enterprise. Traditionally emergency calling is based on the emer- gency responder knowing the physical location of the TDM connection from which the call is coming. With a SIP trunk, that relationship between the physical location and the calling number no longer exists. 106 SIP Trunking Options for handling emergency calling include Continuing to route emergency calls through your TDM PSTN gateways Having a small number of TDM trunks dedicated to this function at the physical location of the service provider Adopting a SIP-based emergency calling solution All SIP trunk providers should provide clear explanations of their solution for providing emergency calling when an IP connection is evaluated. Some aspects to emergency call- ing have not been solved technologically or with the currently offered services. In the United States, the Federal Communications Commission (FCC) continues to work with the industry to define E911 operation, and a geolocation SIP header is in an