FAX Issues

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Asterisk Users 12 Comments


I would like to make sure I got it right:

1. Asterisk 1.4 doesn’t support FAX support. It do however works if u sent
fax from the PSTN and have anther FAX machine answer to it even if it is
behind asterisk. This works like any regular phone, and as far as I know
this mode known as T.38 pass through.
2. If u want asterisk 1.4 to able to sent and receive emails you will have
to patch the source code using the spandsp patches. There are some other
ways to make this work like using IAX, hylafax but I need to know if this
is true. This mode is what know to be as NVFAX detected

3. Now version 1.6 support Fax in a better way then 1.4. There is app_fax.c
in the source code. Can someone please tell me what does this apps do?

4. Version 1.8 as two modes: SpanDSP mode and some other method, what is
the different between then…. can someone please help?


12 thoughts on - FAX Issues

  • No its not T.38 pass through. T.38 pass through is only if the 2 end
    points negotiated T.38, i.e. a provider that supports T.38 and an ATA
    that supports T.38. What you are describing will most likely fail if
    there is any VoIP in between, and should succeed over pure TDM using
    digium or similar cards.

    asterisk doesn’t have the ability to send or receive emails. It does
    have the ability to use something like sendmail to send/receive
    I’m assuming its a typo and you meant faxes.
    What exactly are you trying to accomplish? If you need to know
    something go ahead and try/test it and report back. After all that is
    what we all did to be able to answer it. Apparently the fact that
    others tried it and told you that it works still warrants a
    question:”I need to know if this is true”

    I assume you meant than not then, but I’m not sure if its a typo, you
    might just not know.
    app_fax.c does faxing. The question is what are YOU trying to do?

    Whats the other mode?

  • Fax was never officially supported by Asterisk/Digium using TDM cards.

    I works for the most part, but there is no comparison to a pure POTS
    line. Much depends on the line quality, the sending and receiving fax
    devices. Some places with good fax machines, PRIs coming in and some
    FXS ports would work just fine.

    Others, are a nightmare and make you look bad.

    Depending on your customer’s fax needs, sometimes it is better to tell
    them to keep or have some POTS lines provisioned for reliable faxing
    and to back that up, bring up being able to reach 911 if the phone
    system is down and the company’s liability, even though everyone has
    cell phones. This is the best for TDM faxing with nothing fancy.

    I have sent people to http://www.trustfax.com

    TDM faxing is something that burned me more than once, so I don’t suggest it.

    I have been advised that everyone must post their solutions to the
    list so that you don’t have to test. I think it was a self appointed
    list moderator. People are required to come back, aknowledge that
    something worked and give credit. Flaw: Just because something worked
    for someone doesn’t mean that your solution wouldn’t work better.

    Anyways, I am with you. I learned this stuff with very little
    documentation except .conf examples and code. Integration is where
    things get really fun and creative. Logically testing and failing and
    testing until you find your solution is extremely rewarding (at least
    to me). Making a Definity G3 and Asterisk fully integrate can cause
    severe rage followed by euphoria, so be warned. Being creative is
    dangerous to some on the list.

    Try iaxmodem and hylafax. Alex B did a very nice writeup on how to
    set that up so that it works very well.

    The best mode for T38 is probably Freeswitch or Callweaver.

  • Unless your network is under load and you are seeing dropped packets
    and high jitter, I would absolutely not do T.38. The cheapest and
    easiest approach that I have found is to buy yourself an FXS gateway
    and just make sure you are using ulaw.

    Is this for personal use, or for business use? One feature that you
    may want to utilize is fax tone detection if you only have a single
    incoming line. If you use fax tone detection you can send the call
    directly to your FXS gateway.

    I’ve successfully faxed through my SIP provider as well as through an
    FXO gateway going to a DOCSIS modem (consumer grade “digital voice”).
    TDM and POTS are really synonymous by the way, TDM is just a more
    specific acronym (time-division multiplexing). You need not worry
    about the details though, if you are simply doing inband passthrough
    and you are using 711u on your VOIP legs you should be fine… YMMV of


    On Tue, Aug 9, 2011 at 6:00 PM, Steve Totaro

  • Ok,


    But what is the NVFAX? Does 1.4 support getting faxes per extension?

    In my office I have 1000 ext, each users has it’s own DID number.
    What I would like is that each user can get a fax using his own number.

    I’m running version 1.4.29 under Debian, and FreePBX under a sip trunk.
    It working great, no issues.
    But I can’t get FAXs since the NVFAX option is gray out.

    I do not have a app_fax.so under the modules, and google it I saw you will
    need to patch the code of asterisk using the spandsp tool.

    There is also a stand alone version of this, where no patching is needed,
    but I still want to know what FAX options is supported under version 1.4.

    In 1.6 the app_fax.so comes by default, but I need to upgrade the server,
    which might be good idea, but I’m not sure I want to get into this now.

    Version 1.8 seems to have digum support and spandsp support for faxes, but
    I’m not sure what does it mean….

    All I want is that each user has his own fax number, so when no answering,
    and voice mail is off (can I keep it on?), once fax is coming in, it will be
    converted to pdf and sent to the user mailbox.

    For sending faxes, I guess I can used sendmail with IAXmodem and some faxing

    Thank you all

  • Could not agree more. I have been using a box with an Eicon Diva Server
    card, asterisk 1.4, chan_capi, iaxmodem & hylafax for many years and it
    has worked flawlessly. From a laptop in the field I can send a fax (via
    a VPN) straight from OpenOffice and receive faxes via email.


  • DIDs are cheap, get 1000 extra DIDs specific to that user’s fax. It
    is classy to have your own fax, more secure which was my angle, no TS
    documents sitting on a FAX machine. You can use the same number why?
    Probably only a fraction would need their own fax anyways.

    Grayed out means dependency issues, resolve them and recompile.

    HylaFax and faxdetect. You for the number of phones you are talking
    about, I would use eth2 to connect directly to your Hylafax server
    using IAXModem.

  • SLIN and Alaw work just fine. Again, where are we referring to, the
    SIP provider or the LAN, something you can control or not?

    If faxing is not important then any solution is fine.

    Asterisk has this built in.

    Do you mean a cable modem? They love to use “Digital Voice” when it
    is just VoIP.

    POTS is not use TDM.

    A T1 would use TDM to MUX 23(4) calls onto a two pair. POTS is a
    single pair and comes off some sort of TDM mux device somewhere.

    I would never suggest this in a million years without T38 that isn’t borken.

  • Then you should be fired for not controlling your LAN.

    I don’t care what you doubt or not. I engineered it, Got the
    contracts signed, provisioned, setup BGP, got all that cleared with
    the steering committee.

    I do not assume anything about anyone and cannot define a typical
    user. That is making as ass out of you and me.

    I encourage everyone to attempt and test everything! Not put it in
    production without some reasonable testing.

    Roger That. That is why I advise clients it is not the best way to go.

    He came in giving no details about his setup. Now I see it is 1000 phones.

    I guess you don’t care to read threads, and like to jump in the middle
    not knowing what you are talking about.

    I said that there is really no good way to handle faxes with Asterisk
    except with a T1, IAXmodem, and Hylafax.

    I bought fax for Asterisk when migrating to an all SIP world and it
    was a dismal failure and Digium didn’t even have anyone that could
    explain any of the stats or problems I had. I paid for it so I didn’t
    have to screw around and finally I just took a refund and a big dent
    as far as the companies opinion of me.

    I wanted to try the 1.2 asterisk fork and that other project that came
    from the fact that Asterisk has deadlock issues and a whole cross
    platform rewrite has been done.

    Steve Totaro

  • On Tue, Aug 9, 2011 at 6:00 PM, Steve Totaro

    Wow, I cant believe that someone else on this list is admitting to the
    orgasm integrating a Definity with asterisk can give one. But it takes
    so much foreplay and believe me its worth it. 🙂

  • I do love this stuff. Being self taught, Asterisk actually made me
    learn LAMP from the command line. I was a pure Windows and Cisco guy,
    all self taught since the VIC20 and Commodore 64, I taught myself

    Anyways, just like anything you need to define the job before you
    attempt to choose the proper tool(s).

    Just a quick correction to number 5, Hylafax is a very powerful fax
    app, IAXmodem is just a bit of glue. It has error correction and
    other features that help in TX/RX faxes successfully. Asterisk is not
    the endpoint.

    It has been in dev since the early 90s, starting with SGI and their
    IRIX servers. (I love SGI and the Cray Numalink)


  • Find a carrier, likely a CLEC, willing and able to quickly re-route numbers when you have an outage. This is telecom, you are going to have an outages caused by something totally out of your control. If you have a TN down for whatever reason, having a POTS fax machine or a couple of POTS lines into your PBX for your carrier to re-route the non-working TN to can save the day. Carriers use a variety of methods to give you dialtone on a pair of wires. Make sure the line is plain old boring analog all the way to your carrier’s switch.

    Here is how I look at it. Assume 1 in 10 faxes fail when using ulaw with SIP on a nice stable QoS’d connection to your carrier . There is no specific percentage, read the mailing list archives; I don’t think it is an outrageous assumption. Error Correction (ECM) can only do so much. For personal use, that might not be a big deal. For business use, it is likely to be a big deal. People get very annoyed when they don’t get their faxes. If you don’t need a large number of fax numbers and don’t need to handle a large number of faxes, then use standalone POTS lines and fax machines. It is simple, reliable, and people are familiar with it. POTS is generally easy to troubleshoot, if the telco who provides the POTS service tells you it is an inside wiring issue or a fax machine issue, then chances are it is an inside wiring issue, PBX issue, handset issue, or fax machine issue.

    If you need a large number of fax numbers or need to handle a large number of faxes then you really should consider a PRI to back up your VoIP service — use the PRI for large numbers of fax numbers pointing to app_fax, simple fax to PDF conversion scripts are on voip-info.org, e-mail it to a destination e-mail address. This is quite reliable, relatively easy to implement, uses well known, reasonably mature technology, and protocols. You can use something like NVFaxDetect or the built-in fax detection of DAHDI to do “combined voice/fax” telephone numbers. Enough people listen for the far end fax tone before pressing Send, iy can become an issue. TNs on PRIs are often very cheap compared to DIDs on VoIP, consider a dedicated fax TN for each person and avoid the hassle of fax detecting. There is a large community base and lots of documentation. If you are budgeting for a PRI then a POTS line is not going to be a large expense, might as well have one or two around when all else fails.

    T.38 is a fairly new protocol compared to POTS and PRI and even Asterisk and app_fax. The support community is much smaller, documentation is not as complete. You will likely need plenty of T.38 support from your carrier to get it working. If you need large numbers of fax numbers across a large area, or if you need fax numbers in places you have no presence , or any number of reasons, investigate T.38. Today T.38 seems to require a significant investment in research, trials, and failures compared to POTS or PRI fax. You need to decide if the advantages of T.38 are worth the investment in time.

    Others will have to comment on the option of Hylafax, IAXmodem, or BRI for fax.

    * The opinions above are my own.

  • Hi,

    I’m fighting with this since some time too.

    My experience is, that faxing over IAXModem is not reliable.

    So I’m going with T38Modem, my provider also supports T38 and finally I
    got Mail-2-Fax with T38Modem, HylaFax and Asterisk 1.4 runnig.

    I still have some issues with Fax-2-Mail, I had to patch T38Modem to get
    it working with Asterisk and it only works with Asterisk 1.8.

    But anyway I think it’s better using T38Modem with T38 fax, than using
    IAXModem that uses G711 and which often makes problems with sending faxes.