Int 6B Fn 0600  - Novell Nasi/nacs, Ungermann-bass Net One Serial I/o - C  [S]

   AX = 0600h
   CX = command
       02h send break
       04h disconnect
       06h hold

Return: CF clear if successful
       AL = 00h
   CF set on error
       AX < 0

Note:  this function is also supported by TelAPI and NPC NCSI