The modern telephone system is designed to carry the sound of a human voice talking normally. The engineers save money by cutting out the high a low pitch sounds that occur only in music, or sounds that last for a very short period of time.
Modems convert data into sound so that it can be sent through the phone system.
Just as the telegraph sent data by alternating dots and dashes, the earliest modems sent data by alternating the sound on a phone between two tones. However, modern modems move much larger amounts of data by sending combinations of tones at different frequencies. If you have access to all possible frequencies, there is an enormous amount of data you can carry. Each TV channel is about 3 Megabits of data per second, and a cable system carries nearly 100 channels. However, the telephone system is designed to only handle the frequencies of the human voice, and the maximum amount of data it can theoretically carry is 4000 bytes per second per phone line.