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If an analog telephone adapter is advertised as having 4 FXS ports, what does that mean to me? Can I connect 4 different phones to it? If it doesn't have an FXO port, can I connect it to the public telephone network?


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FYI @TTT if you register your account, you'll be able to access all of your questions, leave comments, etc. There's no need to re-ask questions you've already asked. It looks like you accidentally lost your old unregistered account, so I merged them back together for you. In the future, if you need to clarify, just edit your existing question. – nhinkle Jul 11 '12 at 17:45

4 FXS ports means you can connect it to four regular analog phone lines. FXO ports connect to analog telephones and other devices that connect to regular analog phone lines.

You can make a call if you have a phone and a port to connect it to. For example, if you have a VOIP phone and and an IP connection to the same network as the PBX, you can make calls. If you have an analog phone and some other device provides FXS ports, you can make calls.

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Let's talk about phones. (trust me, this normally isn't something you want to do - the analogue telephone system is old and many of the terms don't make sense in the digital world)

There are two different sides to any given telephone connection. Now, you might be tempted to say that those two ends are the two different people, but the way that old-school telecom people view it, any given telephone call consists of multiple different connections. Two of those connections are the important ones that you interact with: the connection between the caller and the exchange on each side of the call. Basically a phone call looks like this:

Caller --> Exchange --> Telephone System Magic --> Exchange --> Caller

When you're setting up a PBX system, you're dealing with only the connection between exchange and caller, because you're inserting your PBX in the middle of that connection.

Now, the exchange and caller actually do much different things with their ends of the phone line. The biggest difference is that the exchange puts power on the line (around 50v depending on what the line is doing), while the caller's phone does not (in fact, older phones powered themselves entirely from this voltage, which is why they didn't need a seperate power connection).

So, what's with this FXS/FXO business? FXO stands for Foreign eXchange Office, while FXS stands for Foreign eXchange Subscriber. Imagine for a moment that between the exchange and the caller there is one long telephone line with a wall jack at each end (one in the exchange office and one in your house). These names refer to the ports at either end of that line. what's tricky, though, is that they work the opposite of the way you might think, because the names refer to where the port goes rather than where it is. So the jack in the exchange office is called the Foreign Exchange Subscriber or FXS port (because it goes out to the subscriber), and the one in your house is called the Foreign Exchange Office port, because it connects to the exchange office.

So, it looks like this:

Exchange FXS---FXO Caller

When you have a PBX system, you put it in the middle in between the exchange and the caller, so -- remember that the ports are named based on where they go to -- we get this:

Exchange FXS---FXO PBX FXS---FXO Caller

Now, a quick bit of terminology: when we talk about PBX systems, we refer to the exchange of your telephone service provider (and your connection to the world) as the Publicly Switched Telephone Network, or PSTN.

So, when you hear about a PBX system or a telephone adapter having FXO and FXS interfaces, think about it in terms of what you will connect it to. So, to answer your question, a device with 4 FXS ports can be connected to 4 different subscriber phones (or phone lines with multiple phones on them that all ring at once). With no FXO ports, the device cannot be connected to another exchange - so it cannot connect directly to the PSTN.

Does this mean that you can't receive calls from outside or call out to the outside world? No, it actually doesn't, because there's more than one way to skin a cat connect to the PSTN. Here are your options:

  • Buy another piece of hardware with FXO ports
  • Use a different type of subscriber connection, like a T1 line.
  • Connect via a VoIP service over the internet, which based on your tags I think is your plan

So, to confirm what I think you're asking, if you plan on using a VoIP connection to the PSTN you do not need any FXO ports, because you won't be connecting to an exchange in that way. Instead, you'll have your adapter connected to a computer that connects over the internet to the PSTN, or you may have an integrated PBX server that will connect itself. Your VoIP provider will then handle all of the analogue parts of connecting to the PSTN. Your four FXS ports will allow you to connect up four analogue telephone lines inside your organization.

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This site seems to explain what FXS/FXO means, specifically the FXS & FXO & VOIP section.

In relation to your question, a two port FXS would mean you could connect to two FXS gateways, while being able to connect two FXO ports.

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You may also check this video on what FXO is. It is based on Ozeki Phone System XE and it fairly explains FXO and VoIP:


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Welcome to Super User! The link is not a video. Also, link answers are not encouraged, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – Peachy Nov 9 '12 at 15:02

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