Buy a phonecard called the Eurocity card. It is really cheap to call anywhere in Europe or the US, and can be used at home/office or at the phone booth. You can buy them at pretty much any of the Tabacco shops. For 6 euros you can call for like 120 minutes, and there is no connection fee.
That sounds like a great deal. Can it also be used if you take a Spanish móvil elsewhere in Europe (specifically the U.K.)? Or is it only good for calls originating on Spanish networks?