INT 6B - NPC NCSI EXTENDED SERIAL I/O - VIRTUAL CIRCUIT STATUS
        AH = 12h
        AL = virtual circuit number
        CL = ???
        ES:BX -> ???
Return: ???
Note:   this function is also supported by TelAPI
SeeAlso: AH=10h,AH=15h,AH=1Ah,AH=1Bh,AH=1Fh

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