What is "kewlstart"

Kewlstart is a coined term for an extension of loop start ( FXO and FXS) signaling which adds disconnect supervision through the use of an open switching interval (OSI). In addition to the operation afforded by loop start, the CO ( central office) signals the terminal (user) end that the distant party has hung up. The CO switch will remove battery voltage from the loop for about 250 ms, within 6 seconds after the far-end party disconnects.

Most real phone switches support this feature. It is generally required for getting hangup notification.

The other mechanism of POTS disconnect supervision is Battery reversal. In order to indicate disconnection of the far-end party, there is a polarity reversal of tip and ring conductors of a telephone port.