Do most fax machines generate a cng tone?

Answer 1

reference: ftp://ftp.faximum.com/pub/faqs/fax/fax-faq unfortunately CNG tones are only required if the fax machine is working in 'autmoatic mode'. If a person is manually faxing the standards do not require a CNG tone to be generated by the fax machine. This causes problems for some voice+fax services that rely on detecting the fax tone to switch their system to fax recipt mode.