Other Signaling
Modern Loop Start Trunks also have methods of Answer supervision and Disconnect supervision to alert the FXO end of the line that the remote party has answered or hung up. Answer supervision usually takes the form of the CO reversing the polarity of the line for the duration of the call when it has been answered. This is called Reverse Battery or Battery reversal. Disconnect supervision can take the form of the polarity reversing back, or removing voltage from the line for a short period of time (Battery drop). Disconnect supervision explicitly provided over an interface using battery drop is known in the Asterisk PBX community as Kewlstart.
Another type of loop signaling is the ability for the customer to signal the CO that they would like to make a second simultaneous call, a three-way conference call, or answer a second incoming call. This signal is called flashing or hook flash, and is performed by opening the loop for a fraction of a second (usually around 600ms) and closing it again. The flash is longer than a rotary dial pulse (sometimes called a 'short flash', see Dial pulsing), and is shorter than a hang-up (on-hook).
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