This happens in both directions so you can have a conversation.

During the 1980s, telephone engineers were experimenting with ways of sending video pictures over normal phone lines.

The trick was to create a new set of pulses, this time for video, which would travel down the wire along with the voice pulses. These new pulses were at such a high frequency, they would not interfere with the voice signal.

Unfortunately the infrastructure was not really in place at the time for video via telephone, but the idea of sending two signals together over a phone line was to have other possibilities.

Because the video pulses were originally designed to carry a lot of information, they were perfect for carrying digital signals, like internet data.