Best thing to do is either take a tri/quad-band GSM phone with you and buy a SIM card over there for whichever carrier has the best int'l rates. OR, do that and get an international calling card. You'd be using your local minutes to using a calling card for anywhere from 4-10 cents a minute.[/b]
Just make sure that if you get the GSM phone here in the States that the service provider (Cingular, T-Mobile, whoever) will UNLOCK the phone so you can use multiple SIM cards. Many times they do not want to do this - Cingular, for example, makes you be a customer for at least 3 months before they will even consider this and even then they give you the run around. As an alternative, you could buy a cheap GSM phone once you get to Spain.
Also, many plans overseas have very cheap or free text messaging, even to international numbers. It's not the same as talking, but you could possibly use texting part of the time just to check in and let people know your okay and it would save you some cash. Sometimes texting internationally is iffy, though, especially if you send a text from a GSM phone overseas to a CDMA phone in the states (Verizon Wireless customer for example). In that case, the text may fails to come though. Just something to keep in mind. Good luck.