Much like in the world of data networking, specialized equipment is required in order to process and properly route telephone calls to the correct switch, and ultimately the recipient. In the world of telephony, calls are routed through the PSTN using high-end switching equipment located at the central offices and facilities of one or more telecommunications carriers. In larger organizations, these same functions are typically handled by specialized hardware systems known as a private branch exchange, more commonly known as a PBX. While a PBX is used to handle the routing of calls within an organization, public telephone switches are used to route calls across the PSTN.
Although the two devices are very similar in terms of the core function they provide (routing calls), they differ greatly in terms of scale and services offered. A public telephone switch at a CO will typically be capable of handle hundreds of thousands of phone lines, while a PBX may support anywhere from a few hundred to a few thousand. Where the public phone switch is primarily focused on the routing of phone calls, a PBX typically provides a variety of value-added features required by businesses including the voice mail, conference calling, and more. When a company implements a PBX, they are effectively setting up their own personal phone switch, which can be configured and administered according to their needs. While this provides organizations with a higher degree of flexibility, it also adds administrative responsibilities. Furthermore, to reach the outside world (lines external to their organization), a connection to the PSTN and ultimately the phone switch of the carrier’s local CO is still required. In most cases, this connection is provided by a dedicated circuit (T1 or T3 for example), depending upon the number of “outside” lines that the company requires.
Note: Although the implementation of a PBX allows an organization to add services or change the internal configuration of their phone system as their needs dictate, the investment in a PBX needs to be closely scrutinized. In most cases, the implementation of a PBX from a particular vendor requires that all associated equipment (such as handsets) be purchased from the same vendor. This can significantly increase the overall cost of the system.