The network that connects most phones together is known as the PSTN (public switched telephone network).
Fixed phone lines are usually copper wirelines which form a circuit between the subscriber and the exchange, although some recent installations may use optical fiber for part of the distance. An analog line typically uses frequencies of 0-3.5 kHz, with frequencies higher than this filtered at the exchange. The analog speech signals are carried over the digital backbone network as a stream of digitally encoded samples at a sample rate of 8 kHz. The frequences above 4 kHz can be utilized for DSL connections.
A line is a single voice communications circuit between the subscriber and the central switching office. A trunk is a single circuit between central offices and may be analog or digital and is transmitted via copper, microwave, or fiber optics. A trunk group is a grouping of identical trunk circuits between two specific central offices.
Automatic telephone systems generally use numeric addresses, more commonly known as telephone numbers. The addressing system often distinguishes local, long-distance and international calls. Local calls are initiated by dialling the local number. A long-distance number is indicated by a long-distance prefix (CCITT recommends “0”) followed by area code and a number local to that area. International phone calls require an international prefix (CCITT recommends “00”) followed by area code and local number. US and Canadian phone systems use “1” as the long distance prefix and “011” for international prefix.
Larger companies and organizations often employ a PABX (Private Automatic Branch Exchange). This is a telephone switch that defines its own local phone number range, which is commonly embedded in a public local phone number range. Some of the largest companies now even have their own internal telephone networks across the country, or even throughout the world, with limited gateways into the PSTN.
Most PSTN systems use analog communication between individual phones and the local switch. If digital communication is used for an individual phone, the system used is usually ISDN (Integrated Services Digital Network).
Between switches in the PSTN, most signalling is now digital using Signalling System 7 (“SS7”).