Our tech support team at VoIP Supply offers great pre- and post-sales support plus provisioning, consultations, configuration, and installation help. We get a lot of VoIP hardware and software questions and would like to share the solutions with everyone.

In previous Mom’s calling Q&A series, we have discussed: What is the most basic setup for a full-featured VoIP system? Today, we have more new real questions and answers from VoIP users just like you.  


How can I connect a PBX to my existing incoming POTS line?

Q: I am an emergency management coordinator and have an in-office phone system that I want to expand. How can I connect a PBX to my existing incoming POTS line?

A: It depends on the phone system you have. We carry a wide range of FXO SIP Gateways that interface with POTS lines. 

FXO Gateways connect your devices to an outside telephone line and support Failover or Fallback in the event of internet failure or a network crash. An FXO gateway can be implemented to provide access to multiple POTS lines.

Click here to check out!



Come back for more VoIP questions and answers next time! If you have VoIP questions to ask us, please submit a technical support ticket or contact our VoIP experts today at (866) 582-8591


Finally, you are ready to buy a VoIP system. But you are worried about an unexpected power outage and poor call quality. These are common fears we have heard from our customers all the time. Don’t worry, we have you covered!

May VoIP system owners overcome these worries simply with a VoIP Gateway. Let me tell you more about it.

How Does VoIP Connect to PSTN? Using a VoIP gateway for failover and PSTN calling

To connect your VoIP system to the PSTN, you will first need a VoIP Gateway. An FXO VoIP Gateway functions as a bridge between your IP network and the PSTN. Depending on where the voice traffic originates from, your FXO gateway will convert the voice traffic into the proper form for receipt by the destination network (IP or PSTN).gateway-guide

  • If the voice traffic is originating from the PSTN, the VoIP gateway will convert the analog voice signal into a digital signal. This digital signal is then compressed using a codec and broken into a series of packets that are transferred across the IP network using a signaling protocol.
  • If the voice traffic is originating from an IP network, the VoIP gateway will decompress the digital packets into a digital signal that is then converted into an analog signal to be sent across the PSTN.

So even when the power goes down or when the Internal call quality is really bad, you will still be able to use the PSTN to make phone calls and keep your business running as usual. Find a whole list of FXO Gateways HERE.

Is this article helpful? You can also download our free VoIP Gateways Guide to learn more at your own pace or call in at 1-800-398-8647 to discuss with our VoIP experts further!

Editor’s Note: This article was originally posted in August 2010 and has been fully updated for accuracy and comprehensiveness in October 2015.

If you are researching into deploying a VoIP Telephony System on your own for the first time, you are probably seeing FXS and FXO acronyms all over the place.  When you’re new to VoIP and learning it can be very difficult. Hopefully I can help to make a little sense of FXS/FXO for you.

Let’s start with FXS Ports

FXS stands for foreign exchange subscriber.  Since that is so clear and makes so much sense I can stop there right? Not! So clearly the actual meaning of these words isn’t going to help us remember or understand what they mean.

FXS is an RJ11 port that connects internally to an analog office phone or fax machine.  Think of the S as meaning a station or a cubicle.  Any FXS port is going to connect to an analog DEVICE and the cable from the port to the device will never leave the building.

Now, on to FXO Ports

FXO stands for foreign exchange office designates a telephone signaling interface that receives POTS (plain old telephone service).

Um… ok. Let’s put it this way, FXO is a port that will connect a device to an outside telephone line.  Think “O” for “Outside”.  Picture an RJ11 wall jack that connects to a box in your basement which is connected to the line from your house to the nearest telephone pole on your street.  Any RJ11 port on an device that is headed towards that wall jack is an FXO port. It connects your device to the “outside” world or your local area telephone “office”.

How about some visual representation?

I know for myself if I can visually see something it really helps to understand a lot better. Check out the video and hopefully it clears up the info above.

In the event of failure, the most critical phone systems should have a failover feature so phone calls or faxes may continue during the down time. You can still make telephone calls via the POTS line.  Also, you can use the FXO port to make free local calls on your POTS line.

FXS and FXO ports can be used on Adapters and also VoIP Gateways.

Michael Graves & Shdow-IMG_5786
Michael Graves, his dog, and 100+ lb. pumpkin. Photo Courtesy: Michael Graves (www.mgraves.org)

Office technology is supposed to make our working lives easier but the overwhelming amount of options makes it difficult just choosing the right device.

So where do you turn and who do you trust when you want to know who’s used this stuff before, what works and what doesn’t, and how much hair will I pull out trying to install this thing?

This is when you seek the guidance of a VoIP blogger like Michael Graves (@mjgraves) who runs the site Graves on SOHO Technology. He tests products geared for the small office / home office (SOHO) user and offers you, the reader, a wealth of detailed product reviews, guides and how-to articles, and great hands-on advice.

Graves explained to us that products under review spend a considerable amount of time in use on his desk before he even thinks about writing about them, so you know you’re getting quality insight. Which is great, but we wanted to know, “Why?” So we asked him a few questions about how it came to be that he created his site with all these great resources and what he thinks about the future of VoIP.

VoIP Supply: Tell us a little bit about who you are and what Graves on SOHO Technology aims to provide for your audience.

Michael Graves: My professional background includes working as a video editor and graphic designer. I also spent a long while on the technology end of the broadcast business for an English company that builds specialty graphics equipment. That work, mostly system integration and technical support, gave me the opportunity to travel extensively, which was great exposure to a variety of corporate IT infrastructures and strategies.

Early in my career I took a bit of time away from television production to write for a magazine. That exercise gave me the confidence that I could go beyond design and layout to actually write content. It was much later that I started the blog as a way to share the experience gained over years of working from my home office.

VS: How did you get started in VoIP?

Michael Graves
“I was one of the earliest people clamoring for an embedded Asterisk appliance. That’s what started me writing about VoIP. “

MG: Working as I did for a UK based firm, with limited US presence, I was based in my home office in Houston. I wasn’t just occasionally working from home as is more commonplace. I was and remain, 100% home office based. That means that I had to come up with my own IT strategies, including telephony. We had four analog phone lines, and watched as the cost went up while their capabilities were flat-lined.

Eventually, I thought that I needed to be able to do more with the funds. That drove me to look at Asterisk in it’s pre-v1.0 days. After some experimentation, I used a small Asterisk installation with some Polycom and snom phones to replace the small Panasonic KSU that was providing our home and office phones.

I was one of the earliest people clamoring for an embedded Asterisk appliance. That’s what started me writing about VoIP. I documented the process of building an Embedded Asterisk server using a Soekris single board PC and the then fledgling Astlinux embedded distribution. I wrote that for Tom’s Hardware (which later became Small Net Builder) late in 2005.

Since I had analog lines I needed FXO interfaces. My experience with early FXO cards was pretty horrible. Even the new Sipura SPA-3000 (I was in the beta program) was less than wonderful. I eventually ported my numbers to an ITSP, putting all of our voice calling over a DSL circuit.

Back then DSL was pretty slow. It took patience to select and tune a router to ensure good voice quality over DSL. The whole process or moving to Asterisk using voice-over-DSL is what drove me to start the blog. I was answering questions in various forums. It seemed like the same handful of questions kept being asked all the time, so I thought I’d carefully write down my experience in a place that I could use as a reference.

VS: What are your favorite things about VoIP, or some of the creative ways you’ve seen people use the technology?

MG: I think it’s wonderful that we can take standards compliant hardware and software to build solutions to real problems. Sometimes it’s driven by saving money, but for me it’s more often driven by the desire to solve a problem in a new and interesting manner.

People have done all kinds of fun and interesting things with VoIP tools. My former employer had to exhibit at a major trade show in Las Vegas every spring. They paid handsomely for the privilege of internet access at the booth in the exhibit hall. Then they paid again to have a voice line dropped into the booth. Then they paid a third time for calling to the UK head office where their team of engineers were busily trying to finish the code that was to be used for the show. It was silly, bit it was the way of the world.

Once they were familiar with what I had done in my home office they asked me to bring a small PC and a couple of SIP phones to the show. We never again had the convention center install the voice circuit. Better yet, we could have multiple simultaneous calls ongoing over the SIP service, including calls from staff in the UK who had a few of their own snom hard phones by then. We had better calls using G.722 based wideband audio, and we paid less. That’s win-win.

Looking to the present-day and forward, I really like that both HDVoice and video are coming into more widespread use. It’s been about to happen for many years, but it’s really picking up momentum now. Even the US mobile carriers are now supporting HDVoice in limited ways.

Graves answers and shares a readers question.
Graves answers and shares a readers question.

VS: What trends are you seeing now and any predictions for the future?

MG: I expect to see growing use of hosted PBXs by small businesses. That’s what I use myself. There’s a chance that Microsoft could step into this space with a new hosted Lync offer.

I don’t think that the desk phone is going away just yet, at least not in the world that I inhabit. I still need to be on the phone even as my desktop reboots.

More use of video. We see it already with Hangouts and various other services. WebRTC has yet to really make a dent into anyone’s business, but it’s a tidal wave that’s going shake things up a lot.

On the other hand, I’m not that enthusiastic about the more costly video conferencing end-points. I think that their days may be numbered. Low-cost end-point psuedo-appliances like Chromebox for Meetings may do well, but it’s too early to tell as yet.

I hope that we see more use of encryption in all aspects of telecom. WebRTC may be a big step forward in that direction.

VS: Writers read a lot. What’s on your reading list; websites you love or books you’d highly recommend?

MG: It was pain making the switch from Google Reader when it was shuttered. These days I meet my morning using Feedly to catch up on the news of the day. While I have a couple hundred feeds collected there, I’d recommend Light Reading, No Jitter and UC Strategies for telecom topics. Also, Dave Michels who also writes at www.talkingpointz.com is someone I really admire. Avaya’s Andrew Prokop has some great insights into technical matters at SIP Adventures.

As for books, right now I’m trying to stay focussed on the Certified Specialist of Wine Study Guide. I’ve just completed a class and I’d like to take the test some time in the next month or two. It’s a grueling test, requiring 75% correct to pass. I’m told that only one-in-three people pass on their first attempt. Everyone needs a hobby, right?

There are two books I read that I’d recommend to anyone; “Exploding the Phone: The Untold Story of the Teenagers and Outlaws who Hacked Ma Bell” by Phil Lapsely and “Perfecting Sound Forever: An Aural History of Recorded Music” by Greg Milner. Both are excellent accounts of the lengthy history of something that most people take for granted.

Questions about VoIP devices and services are regularly submitted to VoIPSupply.com through a technical support ticket or via the “Ask The Expert” tab on our product pages.

We respond to these requests directly but more often than not, this Q & A would be helpful for lots of other folks.

Below are your VoIP questions answered – Real questions from real people just like you.

Q: In case that my Internet Provider or SIP provider fail, has the Cisco SPA232D the survival option through it’s FXO port?

Cisco SPA232D
Cisco SPA232D Analog Telephone Adapter with Built-In DECT

A:   The Cisco SPA232D ATA is an FXS/FXO gateway with DECT so that in addition to using your analog phones and fax machines with VoIP you can, with the addition of an integrated DECT base station, also add up to five Cisco SPA302D handsets to communicate with it.

In regards to your question, the answer is simply “Yes.”

For more detail I will refer to the post of one of our former colleagues who wrote the post, FXS and FXO – You should be in the know:

Some ATA’s also have an FXO port. This connects to the wall jack by the computer to provide failover. In this case, in the event of internet failure, you can still make telephone calls via the POTS line. Also, you can use the FXO port to make free local calls on your POTS line.

FXO ports are most often used on Gateways and PBX’s to support Failover or Fallback as mentioned above.

For more information on all of the FXO control settings, check out the Cisco SPA232D Datasheet.

Cisco SPA232D ATA shown with SPA302D DECT Handset
Cisco SPA232D ATA shown with SPA302D DECT Handset


Stay Tuned

Check back next time for more VoIP Q & A.

Thanks for your questions!