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Asked: 2026-05-10T23:58:00+00:00

What should be the best way to recognize that a number is connected to

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What should be the best way to recognize that a number is connected to a fax without actually sending faxes around ?

I suppose that a short phone call can be made: the goal is to determine if a number that is declared as a fax line is really a fax line, working and available.

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  1. If you can make a phone call, many faxes create a "Fax Identification tone".

    This signal may be sent by the Terminating FAX machine anywhere between 1.8 to 2.5 seconds AFTER answering the call. The CED signal consists of a 2100 Hz tone that is from 2.6 to 4 seconds in duration. The CED tone is useful for disabling any echo cancellers on the line.

    The CED also incorporates a "silent" interval following the 2100 Hz tone. This interval lasts from 55 to 95 (75 +/- 20) mS. Following this interval, the Terminating FAX machine will initiate the Pre-Message Identification procedures by transmitting a 300 BPS "Line Turn-around" preamble.

    Some older faxes do not support that though, no idea how they can be triggered (i.e. by you sending a tone first, but that could be really annoying if you have a phone instead of a fax number)

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