Free World Dialup (FWD) occasionally offers free PSTN calls, but at the moment, they are not providing this service. Unless there are people you want to talk to that are on FWD, there's not really any point in making calls there. You can find out more about Free World Dialup at
http://www.freeworlddialup.com
Depending on your configuration, you might be able to configure Free World Dialup on, say, Gateway 1 of your Line 1 tab and then add something like the following to your dialplan:
<10393,:>xx.<:@gw1>|<10393,:>*xx.<:@gw1>|<10393,:> **xx.<:@gw1>
This would allow you to make Free World Dialup calls by dialing 10393 then the FWD number.
Of course, getting FWD working from one of the Gateway settings has been challenging for some.