Jay R. Ashworth wrote:
My personal experience is that I've never seen a consumer-grade fax machine with send-CNG turned off, and I don't *think* I've ever seen one on which there was a knob *to* turn it off; I would be less sure about fax modems -- those may have a knob, but I would expect it to default on.
On fax modems the way to silent-dial (and I believe that this was a norm from early-on) to to add an "@" at the end of the dialstring: "ATDT5551212@". I would be very surprised to find any modern fax modem that does not have this capability.
I don't know of any specific fax machine that has such a "knob" to turn CNG off. But my contention wasn't that it was consumer-grade fax machines that were the main culprit here, but rather fax servers (PCs with fax modems in them). And depending on what industry you are sampling, those may actually consitute a fair amount of the caller pool. (For example, some industry software - like insurance agent application software - will have built-in fax features that will use the PC's fax modem - and the application vendor may insist on that feature being used.)
I cannot cite specific software that does it, but I suspect that most fax application developers are aware of the ability to silent-dial, and the reasons why it may be employed. As I've said before, one reason, as an example, is to make the modem capable of hearing ringback - so that it knows if the call has been answered or not (which itself is a unreliable endeavor). Another reason is to avoid annoying the receiver moreso on a call to a wrong-number.
It's off the topic of silent dialing but on the topic of this thread... Brother fax machine manuals state that it is possible for them to erroniously detect certain voices or music as CNG tones, and if that becomes a problem to disable fax detection. And that's basically another point along the lines that say that fax detection is not completely reliable.
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