Fax with FreePBX


Can any DID line support faxing if I terminate the faxes on my FreePBX box or does my DID provider need to have some capability for this? Does faxing work on Asteriak 1.2 or is 1.4 the only version that works? Can someone recommend a reasonably priced DID provider that supports faxing?


In general, Voip, even if you are using a Voice T1, does not work reliably with fax…at all. Not one bit. Some people can send/receive 85% of their faxes a day, others have much more issues. I suggest you get an analog line for faxing. But if you can’t, ipcomms.net has t.38 support for faxing.

In general, yes.

I have sent faxes over voip lines, both from voip line to analogue fax and the other way, and I’ve sent faxes using asterisk connected to a T1 (well, E1 for me) and I’ve had exactly 100% success with it.

I’ve not done more than send a handful of test faxes, only enough to be sure that it works, but it does work.

Now I have no doubt that it’s likely to fail more than an analogue line but I just wanted to say that it can be done. Then there are other options to consider, such as hylafax, T.38, and IAXModem.

If you are using T1 with PRI; then the fax works awesome; We send atleast 300-400 pages everyday and almost 98% make it without any issues.

Here are my 2 cents. to get the faxes to work reliabily

  1. If you are using a SIP DID provider, ; Make sure that you use ulaw and not g729 or gsm; In your free-pbx trunk definition
  2. Check the latency with your SIP/T1 provider; I noticed that fax breakup when the latency goes up beyond 120ms. I recommend broadvoice because they have multiple servers located all over the US; so you could pick the server that is closest to you; You could also try vitelity also. If you are in NY area, you could think about stanaphone also.

Well, ever since you first made this post, I’ve been testing.

If you want to use an outside Voip carrier for faxing to work 100% of the time, use at&t call vantage for 25 dollars a month.

I am actually using them as the primary line in my house for the past 5 years (without asterisk. My home office, however, uses asterisk with aretta communications).

I called call vantage recently and asked how the fax service is. They said it will work 100% of the time as long as there is no internet issues on my end.
So, I hooked up my fax to this at&t line, and started sending and receiving faxes like crazy (I had to turn on fax & modem support in my call vantage account web page). Every fax went through successfully. I’ve make international faxing too and it all went though just like that.
So if you don’t want an analog line for faxing, get att call vantage, but don’t connect it to your asterisk box. Connect directly to your fax. They say you need to use their phone adapter as a primary router in your network, but call them up and tell them you don’t want it like that, and that you want it using LAN, and they’ll provision your device to work properly on LAN.
For your reference, I’m using the dlink adapter they sent me last year. (my linksys adapter I used for 4 years got defected. Plus, I don’t recall being able to successfully send faxes with the linksys adapter)

I’m currently looking into packet8’s faxing service that’s offered. 300 minutes for $10 a month. I’ll post here if I get anything positive from them when I call them tomorrow.


The issue with receiving faxes outside of the compression issue (compressing the signal without using t.38 just does not work well) is the mix of fax machines. The modem signal is a specific frequency with a tight tolerance. If one side is on the low side and the other is on the high side it might not work, that’s why a modern fax machine will start at 33k baud, then step down until it finds a good signal (slow = takes longer) or until 9600 baud fails. That’s why some people had issues years ago with the dirt cheap winmodem cards. We also do faxing via a PRI and a dedicated POTS line on a TDM400 card and out of the last 14 months the only time we had a problem was with one fax machine from a coworkers wife while on vacation in a hotel. other then that it has been perfect over thousands of faxes.