and the amount of equipment required to support this capacity is one of the primary drivers for transitioning to SIP trunking at large headquarters (HQ) locations. A typical large HQ location in the United States can require the capacity of 5,000 simultaneous active calls at a single location to support between 25,000 and 50,000 end users. The cost benefits of SIP trunking at these large locations is compelling because the num- ber of active trunks required per user, to ensure that no users' attempt to make a call, results in all trunks being busy (that is, call blocking), goes down dramatically with high- er amounts of trunking consolidation. When combined with the following benefits, the transition of the HQ to SIP trunking results in a great deal of saving and should occur early in the transition to SIP trunking for an enterprise: Lower number of trunks required per user because of the consolidation 40 SIP Trunking The lower cost per minute of calls and cost per trunk The capability to connect to the SP via an IP link When transitioning the HQ location, the types of calls that should be considered are local, long distance (LD), and international long distance. Most companies, to ensure that the caller id on outbound calls is not changed, use the same SIP trunk provider for both local and LD calls. The decision for which SIP trunk provider should be selected for inter- national LD calls depends on whether these calls make up a substantial portion of the SP charges. Transitioning of outbound and inbound calls at HQ locations requires the porting of a large quantity of DID numbers and can be complex. The time to transition a large num- ber of widely advertised phone numbers from TDM trunks to SIP trunks can be the most complex part of this transition. In summary, you can realize many cost benefits when transitioning to SIP trunks at a large HQ location, and this transition should occur early in the overall transition to SIP trunks for a company. The cost benefits come from several factors, including the consolida