gency call (911) handling. Special handling per Well defined service provider Malicious Call-ID (MCID). Not defined Well defined Multi-Level Priority and Preemption (MLPP). Not defined Well defined Caller-ID delivery. Inconsistent Consistent Voice band data. Modems/Baudot TDD Well defined ill defined or unsupported Fax technology. Requires validation Well defined testing Deterministic traffic engineering. How are bursts SP-dependent Well defined handled? Who generates equipment busy indica- tions, the enterprise, or SP? Who provides announcements? Porting DID numbers. Within single SP control Well defined Geographic and legal dependencies of Independent of Geographically call routing. geography but not of dependent legislation Future rich media services. Great potential No potential Cost to enterprise for service. Inconsistent Well defined Flexibility of call routing; site aggregation. Very flexible SP dependent Security considerations. IP considerations; toll Toll fraud fraud Chapter 11: Deployment Steps and Best Practices 251 However, you must evaluate and assess the feasibility of DID number porting before or during your trial. And, if possible, include at least a few ported DID numbers in your trial to ensure services operate per your expectations. Determine Call Load Balancing and Failover Routing Large sites of enterprise networks usually have a bank of PRIs coming in from the PSTN and the SP load balances or rotates DIDs across all these interfaces. A SIP trunk has a much larger and flexible capacity, but when the SIP trunk is out of capacity or down, it is still necessary to understand what happens to calls arriving during this time: Are these calls deflected to a secondary SIP trunk at a different site, or are they deflected to the PRIs that likely still exist on this site, or do these calls fail as unreachable? When multiple SIP trunks come into the same site, or even different IP addresses belong- ing to the same SIP trunk, you need to understand how calls will be load-balanced or failed-over between t