sentences:  Each should also be preceded with a 2600 Hz "wink", or an MF "8" symbol (900 Hz + 1500 Hz), of about 90 ms in duration, followed by about 60 ms of silence.Most software green boxes emulate this rather than the MF symbols alone.The green box was first proposed in the newsletter TAP in 1981 by Ted Vail and Nick Haflinger.