careful if you are buying used gear. If you have an older Cisco switch and buy a non-Cisco phone, PoE may not work. Similarly, if you have a non-Cisco switch and buy certain Cisco phones, PoE won't work. For example, the Cisco 7960Gs discussed in this chapter support only Cisco prestandard PoE, while the 7971s I'm using support both protocols. Telephony Terms Since VoIP is telephony, there are some terms you're going to need to know. Some of these may seem confusing, but remember, they come from a 100-year-old industry that views us as the enemy at the gates, SO be nice: Private Branch Exchange. A PBX is essentially an office phone system. Automatic Call Distributer. An ACD is the usually large, specialized phone system found in a call center. ACDs are designed to route calls to agents based on special scripts. They can also report call metrics such as average speed of answer, average call handle time, busy hour call volume, and SO on. Pronounced Annie, the Automatic Number Identifier (ANI) is the calling number. It also includes other information that doesn't matter to us in this chapter. ANI is similar to, but not the same as, the caller ID information you see when someone calls you. Even if caller ID is blocked, ANI is present. Pronounced dee-niss, the Dialed Number Identification Service (DNIS) is a code that usually references the number that the caller dialed. Call centers often refer- ence DNIS to route calls. Hairpin Hairpinning a call (sometimes called tromboning) is the act of sending a call or message back from whence it came. Imagine two phones connected to a Call Man- ager Express (CME) system on the same interface. If one phone were to call another and the voice stream needed to traverse the router, the voice stream would go from phone A to the router, then back out the router's same interface to phone B; thus, this call would be hairpinned. This scenario is usually avoided, since the phones usually set up direct communication after call control is completed. While How VolP Works hairpinning is gene