Asterisk and Confused Fax Machines


We have an asterisk box that has been handling our faxes successfully for a few months. However, I have a “report” - one of our sales people got this from one of his customers - that some fax machines get “confused” when they hear our greeting start upon dialing our number. The fax machine then does not send the fax.

I’m not sure I believe this… I wonder if its the fax machine that gets confused or the person making the fax :smile: But I thought I would ask people here if there is a way to make an asterisk DID line beep back at the incoming call like a fax, and if anyone thinks the above claim of confusion has any basis in fax telephony.




If you’re using DID, that is, you have several incoming line phone numbers to the Asterisk box, I’d dedicate one of the numbers to the fax machine, and have it directed to the fax line.

I’m generally opposed to putting fax lines on VOIP systems though. It’s usually a much better configuration to simply leave an analog line off the system, and by itself, to go directly to the fax machine.

Yes, you do end up with one fewer trunk lines on the Asterisk box, but you also gain a loss of power station. Should your Asterisk box lose power, you have a known location to go to that’s not affected by the situation. (Emergency phones should always be available in a central place, and fax machines are usually centrally located in an office.)

If you’re not using DID, and you’re just trying to do fax detect on an ‘S’ extension, it’s really kind of a timing thing. The CNG tones from the sending fax are sent every couple of seconds. Which means that the fax sender could hear a couple of seconds of your main menu greeting before the Asterisk box hears the CNG tones and starts whatever fax handling method you use.

You could try to put this at the begining of your main menu:

exten => s,1,Answer()
exten => s,2,Ringing()
exten => s,3,Wait(2)
exten => s,4,go on to other main menu stuff

This will give all callers a couple of seconds of extra ringing but, more importantly, it will also answer the incoming lines and give the Asterisk box a chance to listen for and hear the CNG tones before it begins to play the IVR greeting. It should switch over to fax receive mode immediately.

Hope that helps.


We do have a spare “emergency” analog line. It was connected to a computer that is running fax software on it. But that system was working sporadically… the PC is very old and having problems. Its still there listening for faxes, but once we started using asterisk handled faxes, people are not wanting to go back :smile:

That system is also connected to a PC, so taht doesn’t solve the power issue really. Also, our Asterisk server is UPS’d to the hilt.

I guess if this fax “confusion” continues to be a problem, I will probably try your 2 second wait before the voice menu starts.

Thanks a lot for the detailed reply!