Fax detect and hangup


We get an annoying amount of fax spam on our phone line.

I don’t understand how fax detect works.
Does it detect an incoming fax without picking up the call?
If so can I detect an incoming fax don’t ring any phones and wait until the spammer hangs up? There is no way I get legit faxes on this number.
The spamer uses anonymous caller id.

(Itzik) #2

Well, that’s an interesting way of sending spam. Why would one go out of their way to send spam via fax?!

But to answer your question, I’d suggest one of the 3 options

  1. As you mentioned, fax detect. You may have to increase the duration time to let the PBX detect the fax, but 4 seconds should work. Set the destination to Terminate Call > Play ringtones to caller until they hang up.

  2. Block anonymous calls, create a new inbound route and set the DID, then in the CID section, enter ‘anonymous’ (without the quotes), and send the call to terminate.

  3. Since they are calling anonymous, your provider or their upstream carrier should be able to see the real caller ID in the SIP headers (you also may be able to see it on your PBX, some providers strip before sending to the end client) and trace down the company this number is with, and report this number to their carrier for abuse. (Make sure it always comes from the same number)


How does faxdetect work?
Will a voice caller notice it?
Is the call still free until i pickup the phone or is the caller billed for just ringing?

(David55) #4

Assuming you are using DAHDI, a fax call is indicated by a pulsed 1100 Hz tone, which will only be received after answer and billing start.

If the call is SIP all the way, I think it may be possible to indicate fax in the initial SDP, but for SIP from the PSTN, I’d assume that the change to the fax codec would only occur as Re-INVITE, after the call is answered. In any case, the PSTN to SIP gateway would need to answer the call to detect the fax tone.


The line is a sip trunk.

(Lorne Gaetz) #6

The channel is answered, and waits for the configured number of seconds to detect an inbound fax tone. If no fax is detected, it follows the normal destination. If a fax is detected it uses the fax destination.

An inbound voice caller will experience a delay, either silence or ringing, depending on how you’ve configured the inbound route.


Thank you for this information.

(Andrew E) #8

I don’t know whether this will help you but we played a similar game a number of months back in that case I knew the number it was coming from (the department of job and family services 2 counties over) calling them to let them know it was the wrong number produced “well we have like 12 departments that use that number to send faxes so we can’t tell you who sent it” great! so they’re going to keep trying (my emphasis) I knew the number and rerouted that particular number back out to our fax number (faxstation) then called and asked for the name of the employee on the incoming fax

basically if you already have a fax machine you could route the incoming faxes to temporarily you might be able to find out what the faxes contain to determine who they are from and either ask them to stop or report them

(Itzik) #9

You don’t need a fax machine for that, FreePBX has free incoming fax to email built in.

(system) closed #10

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