Fax over VoIP Killing Me

I have several Cisco SPA112’s and they’re having issues. We have them configured at our clients sites, with site-to-site VPN’s in place. Phones work great, and these Cisco SPA112’s show as connected properly in the Asterisk Peers, but we can’t get faxes to go through consistently, if much at all.

We’ve successfully received a few faxes (as in 2-3 out of 40), but can’t seem to send any.

I’ve played around with the adapter’s settings, followed various suggested settings, and have set the TX/RX speeds to 9600 and disabled ECM on the fax machines themselves.

Is there a good way to get some feedback from FreePBX/Asterisk?

Depending on your VSP, fax over G711 (it just wont work over compressing codecs like g729) will work in my experience between 98% and 0% ( I get 98% from Vitelity, mostly a lot less than that with other carriers ) If you have a T38 path and T38 pass-though is enabled in asterisk, between your ATA fxs port (your fax machine) and the PSTN (the other fax machine), which needs compliance of both your ATA and your VSP again, then it should be about as good as “good old fax machines”, again about 98%. As to logging, there is little available for analog faxes, If you care to install iaxmodems and/or t38modems and hylafax then there is a lot more logging availabale. (I do and IWFM)


Thanks for the quick reply dicko. We have anveo direct, and they support G711a.

I’m going to give the T38 pass-through a shot, as I’m having no luck elsewhere.

As dicko said not all providers support T.38 so you have to make sure yours does…

I know that SipStation supports it and Flowroute supports it…

I had found another one but unfortunately for some reason it’s as unreliable as G711, maybe even worse…

In the case of Flowroute I know I had to allow re-invites for it to work and I have never tried SipStation…

There are also providers which provide you with their own fax solution like VoIP.ms but as far as I know it is stil beta…

Good luck!


Anvio although great value, will send your call out with no guaranteed T38 conection, no guarantee on fax over G711 either :wink: , a lost packet or two is rarely heard on a voice call, but can and will make your faxes very unusable , I suggest you get another VSP for reliable faxing (or a landline), even if it costs a little more.

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So after setting up T38, I was able to receive faxes at the location, by sending them via faxzero.com. I can’t test sending though right now as I am remote. It showed that it was receiving via T38 as well on the SPA112. What happens when a fax machine without T38 capability tries to send to it?

Few fax machines have inbuilt T38 to get the best of it use T38 pass-through to an ATA that does, the end connection to the fax machine will be G711,

Again, to get better and less complicated fax communication look into spandsp, hylafax and T38modem/iaxmodem as appropriate, I use both, depending . . . ., failed faxes will be re-queued and only the remaining pages will be re-sent, We do thousands of pages a week by that method, some have dozens, even hundreds of pages, complete with fax2email and email2fax , it is not limited to pdf’s either , you can send docx and even mac stuff through libreoffice to hylafax. We rarely have to apologize to the client :wink:


You will definitely need a SIP DiD that supports T.38. I setup fax over VoIP last week. The following website was super helpful:


So, I set routes on anveodirect that support T38, and so far so good!

One question I do have is why I can’t send T38 to T38. When I force the Cisco SPA112 on both ends (send and receive) to be T38, it seems the call is never answered (or perhaps never actually made). But when I set defaults, the sending device goes to G711u and that seems to work.

I guess I’m just trying to understand this and get some consistency. Most of my internet searching hasn’t been very helpful as there are a lot of T38 haters out there.

T38 passthrough need to be enabled in your SIP settings, fax needs to be disabled in your extensions.

With the fax software you speak of, can our users still use their fax machines, or would these be for going all “efax”?

If you have FXS ports, then throw them into the mix too.

The critical issue in Fax over IP with a fax machine is the analog to digital conversion. Many ATAs do a very poor job at that, either because they overdeviate the audio or they introduce other types of distortion. My experience with Cisco ATAs is that they do a poor job at accurately reproducing the audio.

Get rid of the Cisco ATAs and replace them with an Obi 200. I use them all the time for Fax Over IP using G711 with ZERO problems.


Have you looked at our fax solution that solves all the VoIP and T38 issues. http://www.sangoma.com/products/fax-over-ip-service/

Faxing in the analog world doesn’t easily convert analog to digital and vice versa as a voice call does it uses a MOdulateDEModulate process best described at:-


If you understand Modems then you will see why any out of order packets in the stream will break the signalling as they rely on very strict timing of the packet strea. Now ECM is available to correct certain runtime errors but when losses creep past a couple of percent or 20ms of non-sequitor noise at the wrong place then the call will fail on a fax but be hardly noticable for a human ear which has a much better DSP behind it,

Hence T.38 which switches to HDLC coding best described at:-


Which effectively provided mechanisms for out of order packet processing by buffering and general network practices instead of strict analog G711/T.30 this is why you can T.38 fax over compressed codecs.

For a discussion of why fax is hard over IP start and to edumicate the doubters of these problems, Steve Underwood the father of spandsp which is used by Asterisk/FreePBX and many commercial fax machines, offers on Steve’s site:-




I gave up long ago on the “white whale” of working Fax over IP even with T.38. It’s so hit and miss.
the problem is that when you are about to give up entirely, you’ll have one or two work - and it keep bringing you back into it - thinking that you might have figured it out…

I have switched all of my customers over to Vitelity vFAX: http://www.vitelity.com/services_vfax/
This service is rock solid, they give you an “ATA” which really intercepts the fax messages, converts them to TIFF and sends them up to a server via HTTPS where they get faxed out on the PSTN. Works quite brilliantly, and saves you from having to retrain your customers on how to use FAX portals from their PC when all they want to do is just use a fax machine like they always did.

The problem with that is you NEED a fax-machine ;-), just like in the 20th century , many people now prefer email and web services. With a mature software like hylafax and an appropriate email2fax script with Avantfax as a front end you would be surprised by how easy the web bit is. You can of course still have as many fax machines as you want for the over-40’s.

Don’t give up, it works as well as vFAX, it is free and if you have more than three faxes a day is far more manageable, you won’t need as many filing cabinets and will save trees at the same time :slight_smile:

In my experience, if your provider is solid, faxing isn’t that tough these days. I have hundreds of faxes per day going through FreePBX straight into email using the built in features. Lots of faxes everyday sent/received using cheapy Grandstream ATA’s with default settings out of the box on the ATA. Can’t remember my exact setup but, it’s always worked really well, at least for the last few years. These customers are mostly hooked up directly to Level3 SIP though and the connection is not being bounced through six resellers of resellers of resellers degrading the connection etc. Level3 does support T.38 on their trunks but, even before we had T.38 on it was working almost flawlessly on both the ATAs and Fax to Email. I have also had good luck with our Vitelity trunks. I use Anveo but can’t say that I have ever tried to push faxes through them.

By the way, this is all on a fairly old version of FreePBX running I think Asterisk 1.8 as the base.

I think the idea of receiving faxes using a fax machine these days is just bizarre, why would anyone want that. If you can convince your customer or users that receiving their faxes by email is a better way, then you only have to deal with outbound faxes using an ATA. Better yet, you can have them setup a Vitelity account to do the outbound faxing if you like, I have done this with a few clients for just outbound and they have been happy campers. Vitelity’s faxing solutions “Just Work”, I have been using them for my own inbound faxing for years and never remember someone telling me they couldn’t send us a fax.

Hope that helps.

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Now you just need email2fax, relatively simple with postfix and hylafax as a back end. Still no machines involved. Works like a champ.

So my fax woes continue… But thank you all for your input.

Like a few of you have mentioned, G711 and T.38 faxing has been very inconsistent. The other day, it seems 90% of faxes were going through. Today I was testing and got less 50%. Of course, this is for a number of factors.

The doctors offices we’re doing VoIP for really prefer using their giant all-in-one machines to send and receive faxes, so it seems an ATA is what we’re going to have to use.

I think I’m going to look into a Level 3 trunk and some different ATA’s. We can choose to only use Level 3 on Anveo, which is a route marked at T.38 capable, but I’m not sure that is the same as getting one directly from Level 3 quality wise. That being said, I have read about a lot of people having bad luck with the Cisco SPA112’s we’re using, and then having much better luck with some other brands/models, using their same SIP trunks.

Funny thing is, a handful of our customers use the same ATA, the same PBX (FreePBX, current distro), and the same trunks we’re providing via Anveo, and never complain.

Oh, how I wish I could get more logical answers in my troubleshooting!

I’m gonna check out Vitelity’s faxing too.

As an optimist, my favorite post is AdHominem’s: “Get rid of the Cisco ATAs and replace them with an Obi 200. I use them all the time for Fax Over IP using G711 with ZERO problems.” I’m definitely buying a couple of these bad boys right now.

Dicko, with hylafax and/or sandsp, this is a purely efax solution correct?

tonyclewis, this may be a good solution too. Thanks.

Thanks again guys for all your input.