Besides DRT, there are other reasons why calls would ring at the prime set.
Let's say the fax machine is connected directly to the line (and doesn't go through the KSU and an ATA). If the line is programmed to Appear Only on one phone and that phone happens to be the prime set for the line, incoming calls on that line will ring at the prime set. If it is the case, you should see "Line000 to prime" on the display of the ringing phone.