You could do what you describe by setting up an Asterisk box but you will need and FXO connection. There were some modem cards that could be converted to FXO cards and that is really what this card
is...
http://www.voipdepot.ca/index.php?main_ ... ucts_id=25
There is little chance that you computer would have one as most of the ones that come with computers are soft modems.
However, this would all seem a bit excessive to connect the iTouch as the computer would have to be running all the time (consuming lots of power) for it to work.
A better bet might be to buy an ATA box such as the Linksy SPA 3102 that will give you a FXO port and a FXS port. Then the PSTN line connects to the FXO port and you can connect a phone to the FXS port. The box connects to the internet. Now, with the iTouch wireless you can call you the ATA have it pass the call to PSTN and incoming PSTN can be passed via wifi to the itouch. A computer does not then come into the equation.
One problem, I suspect that the software available for the iTouch at present may be a little basic and may have problems reaching the ATA.