The basic principle of the telephone :

The phone is a communication tool used to convey the message sound (especially the message in the form of conversations). most phones operates using the transmission of electrical signals in the telephone network allowing telephone users to communicate with other users.When the receiver was appointed, the position of the phone off the hook. then the circuit divided into two paths which will serve as a positive part Tip which shows zero while in the negative will function as the figures show Ring-48V DC. Both of these pathways that will will process the message from the sender to get to the receiver. In order to produce sound on the phone, the signal transmitted through a cable electrik phone which is then converted into signals that can be heard by telephone receiver.For analog technology, analog signal transmission is sent from central office (CO) will be converted to digital transmission. The figures as  phone number is a specific frequency which has units of Hertz.Primary relationships that exist in the circuit will be on the hook when it opened and then vibration will appear. The sound that appears on the recipient's phone indicating phone is ready to use. 

Source : http://www.scribd.com