When I did my backpacking tour around Europe 6 years ago, public phones were everywhere, and made it easy for me to call home.

Now my parents are going to Spain, and they want to know how they can call me from Spain. Are public phones still prevalent in Spain, or are they going extinct like they are in the USA?

Alternatively, would they be able to use their GSM cell phone to make a call? They are on the AT&T GoPhone plan.

Your parents should call their mobile phone service provider and ask them about international roaming plans.

Will they have a personal computer with them? The cheapest way is to use Skype from an internet cafe - put the Skype software on their notebook and on your computer at home, and it's a free VOIP call to your computer.

Are they staying at a hotel ? If so, the cheapest way is for them to buy a phone card once they are there (from any newstand) . With it they can call from the phone in their hotel room. Thee cards are usually 5 Euros and you get a bunch of minutes to the US (usually more than 300 mins).

Gophone route: Is roaming even available on GoPhone plan outside U.S. and Mexico?

Skype route: If they are taking a laptop, this option should be enabled even if they use other method. There is no additional charge to do computer/computer communication -- provided they have a broadband access. I also have Skype out credit to be able to do computer to regular phone connection.

I also use a pin based local international phone card. As cruiseluv indicated, they give lots of minutes for just few Euros. I use this option when I stay in a place without a broadband access and have to make a lengthy call back to the U.S.

I also have an unlocked quad band gsm phone I use with a local SIM card. This is great for local calls and short international calls. I also use the cell phone I can convince someone from the U.S. to call me - no cost to me, high cost to the caller.

I bought a Verizon phone card at Costco, but you can buy them in most pharmacy stores. I bought a $25 card and it costs .69 cents per minute to call USA.
There is an access number to call and a pin number after that it is the same as calling in the US.
It is simple and easy to buy.
As stated abiove if you travel with a laptop that is the best way.

You can buy a Tarjeta Telefonica at most Tabacs: varying denominations (7-Euro, 15-Euro, etc. the latter may include 'bonus' minutes). You use the card at the Blue Kiosks. I bought the card at a Newspaper & Magazine stand in Sevilla's Santa Justa Renfe Station. I called Phila. and NYC using my Tarjeta Telefonica and the calls were cheaper than calling a relative's Movil in Huelva (80 km. West).

The Technology is identical to that on Eircom Phone Cards in the Irish Republic, but the cards are not interchangeable.