r 67 E1s down. It is therefore imperative to design redundancy and availability measures into SIP trunk access, especially for central- ized or aggregated designs. The failover call routing mechanism also affects your SP because DID calls must be deflected to a different geographical site's SIP trunk entry point when the primary SIP trunk access is unavailable. Cisco Survivable Remote Site Telephony (SRST) is widely used for branch office IP phone survivability and continues to play this role in networks using SIP trunks for PSTN access. If the SIP trunk is centralized and therefore out of reach during failover (because it enters the network at a site distant from the SRST site), SRST alone cannot provide PSTN call routing for the site isolated by the network or device failure. For continued PSTN access, the SRST site either requires backup TDM trunks, legal sanctioning of using employees' mobile phones for PSTN access during failover, or a distributed SIP trunk directly to the site. Mobile phone fallback might or might not be a legal form of providing business emer- gency call access depending on the geography where the site operates-check your local regulations for more information on this situation. High availability SIP trunk network design techniques are discussed in more detail in the next chapter. Tip Design a SIP trunk with high availability along with the SP that you select for SIP trunk service. The more simultaneous channels a SIP trunk carries, the more imperative it becomes that you have solid failover alternatives. Emergency Call Routing Most Tier 1 SIP trunk providers also provide emergency call routing, but you should investigate if this service is available, and if so, how it operates for calls from each site. One alternative is to leave emergency call routing on the TDM PSTN trunks that are already in each branch office and to use SIP trunking initially only for nonemergency 94 SIP Trunking calls until SIP trunk offerings become more mature and all offer comparable basic services. If you decide 