Fax signal to fax machine


I have analog line which is connected to my fax machine (xerox pe120). I connected zaptel card (asterisk) to ext output of fax machine:

When somebody calls that line FaxMachine redirects call to asterisk. Asterisk answers.
It works fine.
Now i want to be able to receive both fax and normall calls on this line.
In FaxMachine manual i read that i can set ANS/FAX mode which allows for receiving fax and normal voice calls. In such situation user of external phone (asterisk here) answers all calls and if he hears fax signal he types 9 on phone (tone dialing) which gives fax signal, which activates fax machine which will start to receive fax.
How can i configure calling 9 in asterisk ? Is not * a special character ?
Did somebody do something similar ?


First of all, read user’s manual carefully because most modern FAX machines has a silent answer feature that will not ring the ext phone if incoming call is FAX, so no 9 is needed.

Secondly, * doesn’t have to be a special character. If it gets in the way, see features.conf to take out anything that uses it.

You said that most modern fax machines will not ring ext phone if it detects fax signal.
But how can it detect fax signal before answering call ? (i think it can not, please correct me if i am wrong). That’s the reason it’s impossible to “hijack” fax session before answering phone. First we have to answer the call (using ext phone) and then fax machine could detect fax signal and receive fax.
My problem is that pe120 has such feature (as it’s written in documentation), but it does not work. When i answer call using ext phone i hear fax signal, but my fax machine can not automatically receive that fax (and can not do it even manually).
Does anybody has any experience and/or suggestions ?


Here’s how a FAX session initiates:

  1. the sending station dials the destination
  2. while waiting for answer, the sending station sends a special tone (FAX tone)
  3. the destination station detects the tone and sends back a high pitched negociation tone.

Now, the FAX tone is sent regardless of line status (answered or not). So a “smart” machine can detect this tone without having to answer the line. That’s called “silent answer” in some documents. This is a feature that needs to be enabled, if included in the machine. (Usually not on by default.)

An alternative technique many FAX machines use is to answer the line then listen for FAX tone if present. If not present, it issues a fake ring - but this usually only works on the attached handset, may not be available for ext line.

If your machine don’t do silent answer, Asterisk can help with 9. You need to make Asterisk detect fax, then.