Jinx,

The circuit you've drawn does not include the DC blocking capacitor.
As drawn there is a DC path through the transformer which will be
interpreted as "off hook".

So yes, you will need to use a pole of the relay to swith in a dc path
for the exchange to detect the off-hook condition.

RP

On 19/11/2007, Jinx <joecolquitt@clear.net.nz> wrote:
> > Since when you close the Relay, the line is shunted by a low
> > resistor, high ring voltage will cease in that moment
>
> I've seen a couple of instances (user circuit and app note)
> showing 220R switched directly across the line. Neither of
> those use an isolating transformer. In my application would
> this mean then a 2PCO with one pole on the line side to
> switch that resistor in ?
>
> > I'd count some rings anyway before closing the relay :)
>
> Yes, that's sensible. Was reading a Scenix app note outlining
> s/w ring detection which rejects noise events, and had planned
> a couple of timed rings
>
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