How do I get dialup idle timeout to work with incoming network pro

My problem is that the idle disconnect feature does not work for me under
Vista Premium.
I get traffic every so often from the internet, and that seems to reset the
timeout.
If I go into the "network connections folder" -> "properties" on my
modem connection -> "options tab" ... I can see a box labeled "idle
threshold" but it is greyed out.

Ideally I would like to find a idle timeout that is after the firewall has
processed the traffic, so that external traffic that is blocked would not be
counted.
or
I would like to know what I need to do to un-grey that box and then be able
to set the threshold higher.

Ideas please.

Other Information:
My idle timeout is set to 5 minutes.
Watching the status of the connection for 10 minutes, I see nothing
outgoing, about 5000 bytes incoming.
I am using an external USRobotics 56K model 325686-03 modem connected to a
serial port.
I have current drivers for the modem from USR, and current firmware in it.
I am using this older modem because it can retain a connection for a longer
time than an newer internal one when doing a long download. Of course then, I
have the problem of getting it to disconnect <smile>

I have tried setting the timeout in Internet Explorer to the same value.