Actually, there is no difference. They are all switches with different capabilities. Let's start by explaining that a switch, more formally a "telecommunications switch," is an electronic piece of equipment designed to accept, queue (hold), and route telephone calls. That's it. Everything else is an application on top of the switch and is driven by software.

A PBX (private branch exchange) describes a phone switch that resides at a company location and has specific software that facilitates both internal and external communications. It comes with built-in "phone company" signaling (ringing, dial tone, busy signals, etc). Depending on the brand and model used, the PBX may be equipped with additional features enabling call transferring, call forwarding, call conferencing, usage reporting, etc. Smaller companies may find they can receive enough PBX-like features by purchasing a KEY System or PC-based telephone system, which are also based on the PBX application.

An ACD can be both a switch and a software application. The application of Automatic Call Distribution is specialized software that enables a switch to accept, queue, deliver, and report on high volumes of telephone calls. This application typically supports additional features such as delay announcements, headset operations at the telephone sets, and real-time CRT displays that report on incoming calls and delay statistics. Many PBX manufacturers offer ACD software as an add-on application to their basic PBX.

A "standalone automatic call distributor" is typically dedicated to supporting a larger organization. Its primary function is to accept high volumes of incoming calls. The overhead of providing PBX-like functionality is minimized, so all available resources within the switch can be focused on answering calls and finding an available agent. A standalone ACD typically supports highly specialized telephone sets, provides sophisticated real-time reporting and monitoring capabilities, and allows for complex routing schemes among dedicated groups of agents. It also captures statistics. In a stand-alone ACD, an adjunct processor does all statistics gathering and reporting. This configuration enables the switch's software to be dedicated only to call processing, which is very important when the ACD is managing hundreds of calls every second.

Lastly, a central office-based ACD is a service provided by your local telephone company. The ACD software and the telecommunications switch are all housed at the local telephone office. The C.O.-based ACD offers many of the standard PBX/ACD features without the capital investment of a switch on premise.