Which device is used to translate digital signals to analog for transmission over traditional phone lines?

The device that translates digital signals to analog for transmission over traditional phone lines is a modem. Modems serve as the critical bridging component between digital devices and the analog infrastructure of telephone networks. When a digital signal, such as that generated by a computer or a network, is to be transmitted over a traditional phone line—which only supports analog signals—the modem converts the digital data into an analog format that can traverse these lines.

This conversion process is essential for enabling internet connectivity via dial-up services and DSL connections, where data must be sent over telephone networks that operate on analog signaling systems. In contrast, other devices listed do not perform this specific function: routers manage traffic between networks, switches connect devices within the same network by forwarding data to the correct destinations, and adapters typically facilitate connections between different types of technology but do not primarily convert signal types in the same way that modems do.