INT 6B - NPC NCSI EXTENDED SERIAL I/O - WRITE DATA ON A VIRTUAL CIRCUIT         
	AH = 18h
	AL = virtual circuit number
	CX = number of characters to send
	ES:BX -> buffer containing characters to be sent
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
SeeAlso: AX=0000h,AH=12h,AH=19h