Int 6B Fn 17  - Npc Ncsi Extended Serial I/o - Disconnect A Virtual Circu  [S]

   AH = 17h
   AL = virtual circuit number

Return: CF clear if successful
   CF set on error
   ???

Note:  this function is also supported by TelAPI, which always returns CF
     clear and AL=00h

See Also: AH=11h,AH=16h