The PF284 can detect the voltage if it is
connected across the bell. Another way is a
small board that Smarthome has. It connects
in series with one leg of the transformer
and detects the current when the bell is
ringing. It too connects to the PF284 acting
as a switch closure. The thing is actually
three of the modules on one board scored so
it can be broken into three. One has
components that where picked to detect a
phone rinning signal and the other two
current like from the bell transformer.