Final answer:
Fire alarm system digital communicators use a modem to send and receive data over telephone lines. The modem converts data for transmission and is specifically suited for phone or cable systems.
Step-by-step explanation:
Fire alarm system digital communicators can send and receive data over telephone lines by using a connecting device identified as a modem. A modem (short for modulator-demodulator) is a hardware device that converts data into a format suitable for a transmission medium so that it can be transmitted from one computer to another. While there are other devices like routers, switches, and hubs that perform different networking tasks, the modem is the one specifically designed to modulate and demodulate electrical signals sent over phone lines or cable systems, enabling efficient data communication for fire alarm systems.