Protocols and standards
CNG and CED tones
CNG and CED are the tones of a fax call: a 1,100 Hz beep repeated by the calling machine, then a continuous 2,100 Hz tone from the one answering.
CNG, the calling tone, lasts half a second every three seconds and signals that a machine, not a person, is on the line. CED, the called station identification, confirms that a fax machine has answered. These are the whistles you hear when you pick up a fax line by mistake.
Why it matters for fax
These tones are not just for the machines. PBXs route calls with them, and voice over IP gateways use them to recognise a fax and switch to a suitable transport. Equipment that misses them treats the fax as a conversation.