Login web GUI，go into“FXS-->Preferences-->Miscellaneous”page，Enable Polarity Reversal

Application scenarios: Polarity (telephone line A, B line is positive and negative) reversed, as to when to reverse: 1. called off hook 2. the end of the call ,hang up.

Polarity reversal will produce a pulse, so logger can record the user's call start time and end time to complete the valuation.

Test Method: You can use a multimeter to measure the polarity of the negative or vice versa from the off-hook calls.The user port to provide a voltage and the meter to start billing, the call ends up and the user port provide a flip Voltage, billing stop billing.