The Telephone Systems are basically telecommunication devices used to send and receive sound commonly human voice over a long distance between people without even meeting each other. Telephone systems use two way communications, means at a time both the people can talk with each other.
These devices use wide range of a worldwide net of telephone lines, fiber optic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, allows any telephone in the world to communicate with any other
All these devices have a speaker to speak in it, a receiver to listen to the voice of other person, a ring system which alerts people whenever a call is coming in, and a dialer keypad, which have numerical digits from 0 to 9 to allow people to dial contact no of other persons at the beginning of every communication. Each telephone system has its unique no. which is used to contact other persons.
All Telephone systems works on principle of conversion of sound waves in electrical signal through speaker and transmit it through telephone lines and at the receiver end the receiver again convert those electrical signal in to sound waves .They work on two way communication principle that both the people can talk to each other simultaneously.
The these devices used telephone lines or wired telephone network to connect with other people from a fixed position, while a mobile phone or cell phone use wireless signals to communicate with each other without having a range limit, the cordless it use a portable handset but can only work within a specified limited range.
All though the earlier Telephone systems used only for voice communication, But with the invention of smart mobile phones it changes drastically as through mobile phones we can text, send audio video files, play games, surf Internet and what not.
This Gadgets also have been changed in itself that now we can use them to have video chats, business meetings, while using VoIP technology with help of computer system, send and receive fax documents by attaching it with Fax machine.
VOIP Telephone systems are also gaining popularity these days. These kinds of devices use an internet connection and a hardware of IP telephone installed on computer. But the IP telephones have two major disadvantages that first they do not have a long power back up in case of emergency and secondly they do not have any fix address as any one changes its location than user have to update its address manually so that no one can found a wrong address in case of emergency.
Articles Source: Telephone Systems: How telephone system work