Telephone is an instrument which reproduce sound at a distance through the help of electricity. It is one of the easiest and common used device of oral communication. It facilitates quick exchange of information through personal talk. There are two types of telephonic communication like external telephonic communication and internal telephonic communication. In the former communication an employee of the organisation talks with another outside the organisational building. In the later communication an employee talks with another within the organisation. Because of this nature of telephonic communication, it has been told that it is quickest and convenient means of external and internal communication. There are three types of telephone connections like direct exchange line, private branch exchange and intercom.
Intercom system of communication has become very popular in our country. It is used to supplement private branch exchange system and operates without an exchange. As this system is not connected with telephone exchange, no permission is needed from the post and telegraph department for the use of intercom service. Multicore cables and telephone receivers are used in this system. Push buttons are used in the receiver set. By putting the button instant connection is obtained with any one of the circuit. If he wants to talk a number of persons at a time he can push the conference key available in the intercom. Today computerised intercom system are developed.