DRPD is a feature available on some fax machines that can tell whether or not to answer the phone depending on the sound of the phone ring.
Some phone providers offer more than one phone number assigned to the same phone line. An example use of this feature would be to have both a business phone number and a home phone number without paying for the extra cost of a full second phone line. Many people also use it to have a fax number on the same line as their regular phone line.
DRPD is helpful as users can get around the problem of having a fax and voicemail on the same phone line – when the fax number rings the voicemail won’t answer, and when the regular phone number rings the fax machine won’t pick up the line before the voicemail.