Phone wire cannot safely transmit Ethernet signals due to differences in wiring standards, insulation, and bandwidth capacity. While both use twisted pairs, phone lines typically have fewer conductors and lack the shielding needed for high-speed data.
Phone lines usually consist of two to four copper wires, while Ethernet cables use eight wires arranged in four twisted pairs. The smaller gauge wire in phone lines can't handle the higher voltages and frequencies of Ethernet signals without significant signal loss., phone wires lack the proper insulation and shielding to prevent electromagnetic interference, which degrades network performance.
Some older buildings still have phone lines installed that aren't being used. These lines might tempt DIY enthusiasts to repurpose them for networking. However, even if you manage to connect devices, the connection will likely be unreliable and slow compared to standard Ethernet.
I once tried using an old phone line to connect a computer to a router in a pinch. The connection worked at a basic level, but speeds were barely usable for web browsing, and video streaming was impossible. The signal dropped frequently, proving that phone wire isn't a viable long-term solution for networking.
If you need to extend your network without running new Ethernet cables, consider powerline adapters, which use your home's electrical wiring to create a network, or Wi-Fi extenders. For a more permanent solution, install Cat 5e or Cat 6 Ethernet cables, which are specifically designed for high-speed data transmission.