🚀 go-pugleaf

I am looking for means of determining if a person has picked answered a
phone (normal analogue).  

As far as I know there are three means for
determining if the phone has been answered.  The first two I have covered,
the last I no nothing about.  

1) Lack of busy or ring signals from a call
progress detector chip.  However this can take upto 5 seconds to determine
there is a lack of 'RING'.  

2) Subscriber Pulse Meter (SPM).  Where
telecom sends a 'charge pulse' every unit interval down the line to
indicate to 'charge' for that interval.  The pulse is usually a burst of
12 or 16kHz tone, and is easily detected by a SPM chip.  This however is
not available in all countries.  

3) Line Reversal?  There is meant to be a
polarity change on the tip and ring inputs as soon as the phone is picked
up?  Does anyone know anything about this and/or how to measure it?  Also
may not be available in all countries?  

Any help appreciated,

Damian