Analog Gateways for VoIP: Understanding FXS and FXO Ports for Seamless Phone System Upgrades

Most businesses don’t realize they’re still using analog phones - even in 2025. Fax machines, old desk phones, alarm systems, and nurse call stations still plug into wall jacks that haven’t changed since the 1980s. But if your internet connection is your main phone line now, how do those analog devices keep working? The answer lies in two simple but powerful ports: FXS and FXO.

What FXS and FXO Ports Actually Do

Think of FXS and FXO as translators between two worlds: the old analog phone system (PSTN) and today’s digital VoIP networks. They’re not fancy. No screens. No apps. Just metal pins inside a box that make old phones talk to new ones.

An FXS port (Foreign Exchange Subscriber) is the one that powers your analog phone. It gives dial tone, ringing voltage, and battery power - exactly what a traditional phone expects. If you plug a landline phone into an FXS port, it works just like it did before the internet. This is how you connect analog phones, fax machines, or modems to your VoIP system.

An FXO port (Foreign Exchange Office) does the opposite. It listens to the phone line coming from your phone company. It doesn’t supply power - it just waits for a signal. When the phone company rings, the FXO port hears it and tells your VoIP system: "Someone’s calling." This is how you connect your existing analog phone lines (like your old landline) to your new VoIP phone system.

You don’t need to remember the full names. Just think of it this way: FXS = sends power to phones. FXO = listens to the phone company.

Why You Still Need These Ports in 2025

You might think analog gear is dead. But it’s not. A 2023 study found that 68% of Fortune 500 companies still rely on analog devices - mostly for fax machines, security alarms, and industrial equipment. Hospitals still use analog nurse call buttons. Factories keep old phones on the floor because they’re rugged and reliable.

If you switch to VoIP but rip out all your analog phones, you’re not upgrading - you’re replacing. And that costs money. A single analog nurse station can cost $1,200 to replace. If your hospital has 300 of them? That’s $360,000. That’s why places like Johns Hopkins used FXS gateways to connect 312 legacy nurse stations directly to their new VoIP system - saving over $287,000.

FXO ports serve another critical role: backup. If your internet goes down, your VoIP phones go silent. But if you have an FXO port connected to your old landline, calls can still come in. One network admin in Wisconsin reported his Yeastar FXO gateway switched 12 lines to analog in 4.2 seconds during an internet outage - no dropped calls, no panic.

How FXS and FXO Gateways Work

These devices look like small boxes with RJ11 phone jacks on one side and an Ethernet cable on the other. Inside, they convert analog voice signals into digital data packets using the G.711 codec (64 kbps), then send them over your network using SIP protocol.

Most gateways come with 2, 4, 8, or 16 ports. A small office might need a 4-port FXS gateway to connect four analog phones. A business keeping its landline for backup might get a 2-port FXO gateway. Some models, like the TC Communications TC3848-1, mix both: four FXS ports for phones and one FXO port for PSTN backup.

The conversion isn’t perfect. Analog signals are messy. Digital networks are clean. That mismatch can cause echo, choppy audio, or fax failures. Good gateways fix this with echo cancellation (ERL > 15dB) and T.38 fax protocol support. Without T.38, fax machines often fail. One Reddit user spent weeks troubleshooting until he enabled T.38 and set the jitter buffer to 120ms - something the manual didn’t mention.

FXS vs FXO: Real-World Use Cases

Use FXS when you want to:

• Connect analog phones to your VoIP system
• Keep using fax machines without replacing them
• Integrate alarm systems, elevators, or industrial equipment that only work with analog lines
• Preserve existing hardware during a VoIP transition

Use FXO when you want to:

• Use your existing landline as a backup for VoIP
• Keep your current phone number from your old provider
• Make calls when your internet is down
• Transition slowly - keep PSTN while testing VoIP

You don’t need both unless you’re doing both things. A small business might only need one FXS port to keep their reception phone. A hospital might need 50 FXS ports for nurse stations and two FXO ports for backup lines.

Choosing the Right Gateway

Not all gateways are equal. Here’s what to look for:

• Port count: Match it to your needs. Don’t overbuy.
• T.38 support: Non-negotiable if you use fax.
• Compatibility: Make sure it works with your PBX (3CX, Asterisk, Yeastar).
• Failover speed: Look for gateways that switch to PSTN in under 5 seconds.
• Regulatory compliance: In the U.S., it must meet FCC Part 68 for surge protection.

Popular models include the Yeastar TA400 (4 FXS ports, $150), Patton SmartNode 4120 (for heavy-duty use), and Cisco VG350. Yeastar’s TA series leads in reliability - Network World gave them a 98.7% fax success rate in 2023.

What Can Go Wrong (And How to Fix It)

Even simple setups can fail. Here are the top three issues:

1. No dial tone: Usually means the FXS port isn’t powered or the phone line is reversed. Check polarity. Some lines need a crossover adapter.
2. Fax doesn’t work: Turn on T.38. Set packetization to 20ms. Disable voice compression on that port.
3. Call drops or echo: Adjust impedance settings (600-900 ohms). Enable echo cancellation. Reduce jitter buffer if audio is choppy.

Most problems come from wrong ring detection settings. If your gateway doesn’t hear the ring, it won’t answer. Set it between 70-90V AC. Patton’s support team says 78% of their tickets are from this one setting.

The Future: Is This Tech Going Away?

Yes - eventually. The U.S. plans to shut down its last PSTN lines by 2030. The UK, Germany, Australia, and 25 other countries are already done. By 2027, most of Europe will have no analog lines left.

That doesn’t mean FXS/FXO gateways disappear. It means their role changes. They won’t be for connecting to phone companies anymore. They’ll be for connecting legacy devices that can’t be replaced - medical equipment, factory sensors, fire alarms, and emergency systems. Gartner predicts a $2.8 billion market for these niche uses by 2030.

For now, they’re a bridge. Not a destination. But for businesses still running analog gear, they’re the only bridge that works.

Quick Setup Checklist

• Identify what you’re connecting: analog phones? fax? alarm system? (FXS)
• Are you keeping your landline for backup? (FXO)
• Match port count to your needs - don’t buy more than you need
• Confirm compatibility with your VoIP system (3CX, Asterisk, etc.)
• Enable T.38 if using fax
• Set ring detection to 70-90V AC
• Test with a real phone and fax machine - don’t just rely on software

Frequently Asked Questions