At first your computer must have a Modem. Modem (modulator demodulator) is device that converts data to an analog signal that can be transmitted over telephone lines. The computer’s digital information is transmitted as analog signal. At the receiving end the analog signal is converted to digital information in a readable form.
Modems are available in various forms. The standard interface for connection the external modems to computers is called RS-232 interface. Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all computers have. Some modems come as an expansion board that you can insert into a vacant slot. This type is called onboard or internal modems.
While the Modem interfaces are standardized, a number of different protocols, for formatting data to be transmitted over telephone lines exist. Some like CCITT V.34 are official standards while others have been developed by private companies. Most of the Modems have built-in support for the most common protocols.