12/16Khz and reverse polarity

12/16 Khz signal is sent from Exchange to subscriber premises for around 100ms duration.
In reverse polarity signal potential(-48V) on a & b limbs are reversed momentarily(donot know exact dutration).
Both these signals are generally used to send metering pulses for different machines (One sensing pulses other sensing reversals)

12khz/16khz is sent from the exchange to calling party for billing purpose. Line reversal is polarity reversal on line a and b limbs once the calling party and called party are connected each other (once called party lifts handset)