When you plug a regular landline phone into a wall jack, you’re using an FXS port, a physical interface that delivers power, dial tone, and signaling to analog telephones. Also known as a Foreign Exchange Station port, it’s the bridge between old-school phones and today’s internet-based calling systems.
Most businesses don’t realize they still rely on FXS ports—even if they’ve switched to VoIP. You might have a fax machine, an alarm system, or a receptionist’s old desk phone that only works with analog signals. An FXS port, a physical interface that delivers power, dial tone, and signaling to analog telephones lets those devices talk to a VoIP gateway, a device that converts analog voice signals into digital packets for internet transmission. Without it, your analog phone just sits there, silent. That’s why companies keep FXS ports alive: they’re not obsolete, they’re transitional. And if you’re moving from landlines to VoIP but can’t replace every device overnight, this port is your lifeline.
Here’s how it works: the FXS port supplies 48 volts of DC power to your phone, sends ringing voltage when a call comes in, and detects when you pick up the handset. It’s the same thing your old phone company did—except now, that signal comes from a VoIP adapter, not a copper line. You’ll find FXS ports on devices like ATA adapters, business VoIP phones with analog outputs, and IP PBX systems designed for hybrid setups. They’re not glamorous, but they’re essential for smooth transitions. If you’ve ever wondered why your fax machine still works after switching to Zoom Phone, the answer is almost always an FXS port.
Related concepts like SIP trunk, a virtual connection that carries VoIP calls over the internet between your business and the provider and PSTN integration, the process of linking traditional phone networks with modern IP systems often get all the attention. But without FXS ports, none of it matters to the person trying to use their old phone. This is where theory meets reality. The posts below show you exactly how to set up, troubleshoot, and maximize FXS ports in real-world systems—from small offices using analog phones to call centers that still need fax lines. You’ll find guides on choosing the right VoIP gateway, fixing no-dial-tone issues, and connecting multiple analog devices without buying new hardware. Whether you’re upgrading slowly or keeping legacy gear for compliance, these articles give you the practical steps to make it work.
FXS and FXO ports let you connect analog phones and landlines to modern VoIP systems. Learn how they work, when to use each, and how to avoid common setup mistakes in 2025.
