I agree with Nevado's recommendation about the
Telefonica Phone Cards. As I recall, they come in 6-or-12 Euro denominations. They are available for sale at
Tabacs and many newsstands. You use them at the 'blue' pay phones (found all over the place). When you insert them, the screen on the phone tells you how many Euros credit you have remaining. A cousin told me I could add Euro credits from a phone; but I haven't tried that.
I direct-dialed from Santiago de Compostela to Philadelphia one morning and it cost me about 1.50-Euro for a call of about 10-minutes. Calls within Spain are extremely cheap, unless you are calling someone's Mobile phone. On another trip, a call to my cousin's mobile phone in Spain cost me as much as a direct-dial call to Philadelphia from Spain.
Other Phone Cards require you to dial an '800' number for access and then the calls cost more. I use the Telefonica cards exclusively