Well,
you could make a unit that reads the dialing made by the caller,
and then it would refer to the appropriate charge band.
It could also register the length of the call.
And the time of day, which affects pricing.
But i dont see any way for such a unit to take account of the
users agreement with the telephone company.
I get half price overseas calls included as part of the 'package'
that i am billed.
There are many 'packages' and 'deals' some of them are quite
difficult to follow, to make a unit that would keep a 'running
total' sounds like a difficult proposition to me.
You say you only need to know the point when the called party
answers the call ?
You
may be able to do this with echo.
I may be wrong, but i think the sound changes when the call is
answered. You will have to have a listen to check, but i think
the audio at the calling end changes when the called party
picks up.
Whether or not you can detect that i dont know.
Best of luck with it,
John