ly owned by several SPs in different geographic areas. The DID numbers owned by the SP that you evaluate for SIP trunk service will most likely port these. However, the DID numbers owned by other providers will most likely not be portable. This means one of two actions on your part: Implement distributed SIP trunking to retain the DID numbers that cannot be ported to a centralized SIP trunk architecture. Or if your business case rests heavily on costs savings resulting from centralized trunking, simply leave these sites with nonportable DID numbers on TDM trunking until a later date. Implement centralized SIP trunking and change your DID numbers where they can- not be ported. DID numbers from different countries can most likely not be ported. For example, if you have several offices in both California and Mexico and consider a single centralized SIP trunk out of one your California offices, it is likely that your Mexico offices' DID Chapter 6: SIP Trunking Models 89 numbers will not deliver via that central SIP trunk. You need to put in at least two SIP trunks, one in California and one in Mexico. The other sites within these regions can most likely be aggregated, and numbers might be portable within each region. Tip Evaluate what DID numbers you have in different regions and offices and determine which SPs own these today. Further, determine which DID numbers will adversely impact your business if they change and which ones can change without much impact. Armed with this analysis you can more easily evaluate different SIP trunk SPs' answers to DID portability questions. For example, if you are in the high technology business, your outbound marketing division might be located in a city where SP X serves the DID numbers to the people in this organi- zation. Further, your engineering division might be in a different city in which DID num- bers are served by SP Y. If you start investigating SIP trunking, it is important that SP X preserves and ports the DID numbers for your marketing division, but it might be much l