It’s all a matter of bandwidth (not to be confused with throughput). All digital communications is a combination of a very low voltage typically known as a binary zero and a fixed higher voltage known as one that switch back and forth. Digital is actually a very defined analog signal. The surrounding environment (circuit board, connector or cable) has to be designed to handle a certain amount of bandwidth to reliabily handle digital communications. ANSWER: The phone line is designed only to handle a small frequency bandwidth such as the voice range (300 to 3000 hz). OPTIONAL: Over time we developed methods of how modify the digital signal so it could modulate at a high rate as well as modulate within the gaps of the modulation and invoke phase shifting at the same time with the end result of effectivally carrying the data of the digital communications at real time but as analog within a 300 to 3000 hz range. I hope I didn’t go to far. This was a tough questions and involves a lot of physics and an understanding of data commuications.