this type of solution is that the routing of this call, which is normally based on the number that is the CALLED, needs to change to be based on the CALLING number. This is because there is a single emergency services number, such as 911, that is used throughout a large geographic region, but the call is routed to different locations based on where the call is originated. Table 5-3 Method to Implement Legal Intercept Method Advantage Disadvantage Use PSTN resources No change in implementation Hinders ability to offer new required. services. All calls must go through PSTN. Use new IP-based Legal Allows for new services and Requires new infrastructure to Intercept Server optimized calls. be implemented that can have significant additional costs. Request a waiver No new work to enable SIP Unlikely to be granted. trunk service. 70 SIP Trunking When using SIP trunks for PSTN access, routing the call to the correct PSAP requires that the physical location of the CALLER be known and the CALLED number is convert- ed in what has historically been referred to as a "SELECTIVE ROUTER" to the phone number of the correct PSAP to call. With SIP trunking, as with TDM trunking, this same lookup needs to occur. However, there is a chance, because of the transient nature of IP addresses, that the physical location recorded is no longer correct. As such, a more com- plex system that is defined in the E-911 service requirements for SIP trunk providers required to be implemented in the United States. Other countries have similar require- ments. In summary, in many regions of the world, there are two required supplementary services of SIP trunks for PSTN access that SPs need to consider in their design. These are legal intercept services and emergency calling services. Both of these services have well- defined solutions that can be implemented to ensure that a SIP trunk service can be deployed and complies with appropriate regulations. If an SP designs a system without consideration for these services, it could not be a service that is