Shoutbox

One reason why this won't work is that even if your phone line detected the slightest bit of noise it disconnected (i.e. when the phone was picked up) you wouldn't get a dialling tone as the modem cuts out (you would get the dead line tone or silence) so you would have to put the phone down and pick it back up anyway (maybe a few times).
If the line detected a pulse or tone for a dial (which really causes problems with Dial Ups) it would take about 3 seconds to disconnect anyway.

The only way I could think you could do this is to buy some kind of hub that both the modem and the normal phone are plugged into and as soon as the phone is picked up (and then tries to connect to the local exchange for an outgoing dialling tone) the hub cuts off the internet connection and makes sure that the phone connects to the exchange, if then the computer automatically gets the modem to keep connecting with autodial (despite the hub blocking it) as soon as the phone is put down and the hub allows the modem to reconnect - in theory it should kick in almost instantly for a redial.

- However this isn't software, but it is the only way I can think to do it. The best place to ask would be a phone shop or ideally a BT shop that's near you.

You could in theory set some software up to carefully monitor your phone line and take the appropriate action with the modem when needed, but to get it to work you would have to set it up very carefully to process all sounds being received by the modem and it would likely be a farce setting up as it would have to respond to a very specific noise of the phone being picked up and nothing else. In any case the software (even if someone finds something) won't be very reliable.

The obvious sollution is to get a broadband line, but this is likely not an answer you want, but if you shop around you may be able to find one that works out about the same price as the current dial up, it may be slow broadband with a low cap but for most people it will easily do.

But really is it that hard just to click disconect and reconect every time you want to go off and back on the internet for whatever reason, I know as a previous dial up user how it is, but its the most reliable way to get it to turn off and back on when you want it to.