you pickup line onhook is received and you get dial tone
you dial a number
an ss7 message at the closest stp to you is generated to locate the other phone and see if it is busy
an stp on the far end, checks the line finding it not busy
an ss7 message is passed back to your stp to let you here a ring on your line and it generates a ring on the far end
the far end picks up and the route(circuit) between the 2 lines are connected.
when you hang up the stp sends an ss7 message to the far stp to hangup he call
there is a whole lot more to it but that is the basics of the AIN
thanx,
bill