Depending on the area you are trying to call there are different services that allow free (or really cheap) voip calls to PSTN. VoipBuster as mentioned is a good service and allows free calling to POTS in some major countries as well as Cellular termination in the U.S. and I believe Canada. Last I knew you need to activate the account by purchasing 1GBP worth of services which I think the current exchange rat is 1 GBP = $1,337 USD or something in that range. The idea is that once you have activated your account you will have free termination in a number of countries with a limit on weekly usage that iirc is around 300 minutes. As of this last week or so you can purchase Skype unlimited in the U.S. and Canada for $2.95 a month and you no longer need to buy it 5 months at a time so it's just $3
Free calling is great and everything, but it's honestly much less hassle to spend $1-3 and get legitimate service with little effort.
As far as dialing with VoIPbuster you do need to dial your country code, if you can't figure out how to dial the country code then please stop what you are doing and give up right now.