Int 6B Fn 18  - Npc Ncsi Extended Serial I/o - Write Data On A Virtual Ci  [S]

   AH = 18h
   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=30h

See Also: AX=0000h,AH=12h,AH=19h