Hardware & Endpoints
VoIP Gateway
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
A VoIP gateway is a box that translates between traditional phone lines and internet calling, letting an older phone system or a set of analog lines keep working on a modern network.
How it works
It sits between the two worlds and converts in both directions. On one side are traditional connections — analog lines, or a digital circuit from a phone company. On the other is your network, speaking SIP. The gateway converts the signaling and the audio so each side sees what it expects.
Which ports you need is the whole specification, and the two names cause endless confusion. An FXS port provides a line, so you plug a telephone into it. An FXO port receives a line, so you plug a phone company's line into it. The way to remember it: FXS feeds phones, FXO is fed by the phone company.
Why it matters
Gateways exist to make change gradual. Replacing a whole phone system in one weekend is disruptive and expensive. A gateway lets an office move to internet calling while keeping an existing PBX, an overhead paging system, or lines that must stay for other reasons.
They are also the practical answer for things that are not phones. Elevator phones, gate intercoms, alarm panels, and card terminals all expect a traditional line, and a gateway supplies one. The important caveat is power: anything on a gateway depends on your building's power and your internet, so safety circuits that must work in a power outage need a deliberate plan rather than an assumption.
FXS ports provide dial tone, so telephones and analog devices plug into them. FXO ports accept dial tone, so incoming phone company lines plug into them. An office keeping old handsets on a new system needs FXS; an office keeping old lines on a new system needs FXO.
Related terms
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What is the difference between a gateway and an ATA?
Scale rather than function. An ATA is a small gateway, usually one or two ports, for connecting a single phone or fax. A gateway generally means a larger unit with many ports, or one handling a digital circuit for a whole building. The conversion job is the same.
Do I need one?
Only if you are keeping something traditional. If you are moving entirely to a hosted phone system with IP phones or apps, you do not. Gateways are for offices keeping an existing PBX, analog handsets, or devices like alarms and elevator phones that expect a real line.
What are FXS and FXO ports?
FXS provides a line for a telephone to plug into. FXO accepts a line from the phone company. Getting this backwards is the most common ordering mistake in the category, and the resulting hardware simply will not work for your situation.
Will my alarm or elevator phone work through one?
Technically yes, but check the rules before doing it. These circuits are often required to work during a power outage, and a gateway needs electricity and internet. Many businesses keep one traditional line for safety circuits and move everything else.