[asterisk-users] Is possible to use FXO Digium card like a Fax modem?

[Kevin Larsen]

> There are also cheap USB fax modems that you can attach to an FXO 
> port and that works fine. All you have to do then is configure 
> asterisk to detect incoming faxes and route them to that port 
> (faxdetect=yes?).
> 
> This worked great for me when I had all my incoming calls coming 
> over a Century Link POTS line. As I approach retirement and want to 
> save money, I switched from the $44/month POTS line to a pennies-
> per-month VOIP service via IAX registration. So now I'm wondering 
> whether this setup would still work. The question undoubtedly shows 
> my ignorance of telephony stuff. I'm willing to do my homework, I 
> just want to know if it's even possible to do this, or if there are 
> better ways to handle fax over VOIP.

I am going to say this with tongue only partially in cheek. The better way 
to do fax over VOIP is not to do it. It is finicky and unless you have a 
real need for it, it isn't worth the time it takes to make it all work. 
Even working, you still have complaints every time a fax fails to send or 
receive as people somehow have this expectation that faxes should never 
fail. To quote the movie War Games, "The only winning move is not to 
play."

It would be preferable to use a scanner and email to send documents if at 
all possible. If you still need the occasional fax, I would recommend 
using a fax service and letting that be someone else's headache.

That said, my company still has plenty of people who insist that faxes are 
the greatest thing since sliced bread, so I get the fun of supporting 
them. Your options, depending on scale are to use one the solutions you 
can integrate right into the Asterisk server or to use an external package 
and then you just forward the calls from your asterisk box over to your 
fax software (this is the one I use).

Make sure that your SIP/IAX provider supports T.38 faxing (specifically 
transcoding) as this will make your life much easier. You have to be 
careful here as many providers will happily pass T.38 along if it comes in 
that way, but if someone with an analog line/fax setup sends you a fax, it 
will hit their system as audio and pass on to you as audio, which with SIP 
can be fraught with danger unless you have a really excellent connection 
to your sip provider. With transcoding, they can convert it as it enters 
their system to T.38 and then just pass the T.38 to you, which results in 
greater successes. T.38 passthrough is common, transcoding less so, but it 
is getting more common as time goes on.

Also, if your provider does not support T.38 transcoding, plan on sticking 
with ulaw or alaw for faxing. The compressed codecs do not allow the audio 
signal to pass properly and faxes will not work. 

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