How to Fax over VoIP on the Internet

Brightstar wrote: “I keep hearing about how hard it is for faxes to work through VoIP (Voice over Internet Protocol). I’ve prepared a top five list of things you can do to make sure your faxes have the best chance of going through.”

  • Set your baud rate to 9600 and turn off Error Correction Mode. The slower your fax goes, the more likely it is to go through without errors. Most VoIP providers are unable to support the default baud of 33600 baud due to the amount of data that has to go through. Error Correction Mode was designed for faxing PSTN to PSTN. Voice over IP often has extra noise added due to jitter instabilities in the connection as it travels from one fax to another. This causes false positives for Error Correction Mode because it will cause the fax to loop over and over trying to fix these noise errors, when it can’t be fixed. This will end in a communication error usually.
  • Use a provider that supports T.38 – T.38 is an audio codec designed for faxing over VoIP. it allows for more detail and better looking images as well as stability by creating redundancy in the RTP stream.
  • Set your packetization to 10 milliseconds or less. The more header there is in the stream, the less likely jitter and instabilities in your internet connection are going to cause a synchronization error. (You may need to contact your provider to make this happen if your box is locked, such as if you use Vonage.)
  • Keep on top of your ISP. Stability in your Internet connection are paramount in making sure that faxes are successful. Speed tests can help you track this. Look for an advanced speed test that offers more than just and upload and download speed. A personal favorite of mine is located at MyVoipSpeed (Note: An up to date Java version is required) which is really nice because once you’re done, you can click on the detailed analysis link and then call up your ISP and give them the URL for the test so they can see what’s going on.
  • When all else fails, call your VoIP provider. There may be issues in the routing of your calls that are causing problems. Only your VoIP provider’s call processing team will be able to fix this kind of thing. Have your provider test from a PSTN line and then their own VoIP line. If they can send through PSTN but not their own VoIP line, it’s a routing issue on their own end.
  • Bonus Tip! Don’t even think about faxing using Voice over IP on a satellite or wireless connection. These kinds of connections are too unpredictable, and add a huge amount of latency that will make it impossible for fax machines to sync up properly.

26 thoughts on “How to Fax over VoIP on the Internet”

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  3. I used to use vonage and also they provide toll free numbers at a great price. im not being paid by them to say this either lol. but i started having probs with the modem going out on the phone and online frequency and not sure if it was the isp or vonage causing the prob ?

  4. I use fax in my business, where it’s necessary. Of course, it’s a fax/scanner/copier (color, of course) and fully controllable from my computer.

    I’d rather use email, and my clients do well with me just sending them their invoices as a PDF in an email. But some third parties will only accept a fax, and for some reason legal documents are only recognized as valid when faxed, not scanned, so they MUST be faxed.

  5. I do not understand why, after so many years, do not have a software, a “driver” to send digital info (fax) by digital line (network, internet)…

    I am able to “print” from Word to a “fax machine”, a digital info, and send it by analog phone line only, but not to send to an IP address or to a “gateway” from internet to a phone line (such technologies were used few years ago, but I did not hear any more these days, when VOIP is more used). At least a “sending” machine for each PC should be easy, up to the moment we will have “answering fax” machines at an internet address…

    Generally, we have software, fax machines, to send or/and receive data, but only as a digital communication over an analog phone line.

    Strangely enought, it should be easier to have digital communications over digital line (network, internet…).

    And I believe we can go higher than 9.6 kbps, like 5-8 MBps or even more…

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  7. I can’t express how much internet fax has helped my business. The price is great, it is reliable, saves a lot of time and leaves me extremely organized. I am also very conscious about the environment so I like how you save paper.

  8. A VOIP provider should be able to tell you if faxing is supported or not. This shouldn’t be a major problem anymore with T.38 and a stable ISP and VOIP service provider.

  9. Disable ECM mode = biggest VoIP myth ever!  It’s like people will “wow a success!” on their machines, but in reality it can be fake.. no Ecm mode means you don’t care if pages may be unreadable.  All in all, you *want* any form of error detection possible (ecm is software level error correction) on a VoIP line.  As for as I know in my various testing “false positives” just do not happen.  “The slower your Fax goes, the more likely it is to go through without errors.” is just not true.  VoIP line “noise” is caused my network jitter, not continuous “analog fuzz”.  In VoIP, your might get several seconds of “good enough” audio to allow a number of sections to go through, such as lines or octets.  If you slow the transmission down, you only better guarantee that a full line or octet will never be complete within the good audio window.  Ecm mode is very good at recovering from jitter induced data loss by asking the sending machine to retransmit the lost data called frames and octetes.  Also generally speaking, slowing the baud rate to 9600 from 14400 isn’t necessary.  Since 9600 and lower do not have modem protocol error correction, any gain you might get is lost due to not having any form of error correction.  14400 is later and has forward error correction in addition to the software level Ecm mode.   …”because it will cause the fax to loop over and over trying to fix these noise errors” – but you *want* Ecm mode to fail rather than to deliver a bunch of blank and unreadable pages!  If your line is that bad to cause Ecm to fail outright, then disabling Ecm mode will certainly give the receiver blank/missing and unreadable pages.

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