> That's not how a printer with a fax/modem works.

It is how at least one of them works, and how I assumed all of them work. http://www.whoopis.com/howtos/laserwriter16-600/LaserWriter_...:

“FaxOptions Dictionary This dictionary contains parameters that direct fax transmissions from a PostScript language program. Refer to “Fax Parameters” on page 29 for further information about FaxOptions defaults and the keys found in this dictionary”

Google tells me this is what you needed in your PostScript file to send a fax:

  <<
    /OutputDevice /Fax
    /FaxOptions <<
      /DialCallee (555-0199)
    >>
  >> setpagedevice