I am not very savvy when it comes to electronics, but if you're a low tech person, my low tech solution might work for you.
In 2000, we bought a cell phone in a Movistar store in Pamplona (they are everywhere, I bet any town over 8,000 has one). The phone was 25E and came with 25E of talk time so it was a pretty good deal. Since then, we have used it on five or six caminos, each time just buying a new card in the Movistar store when we arrived in Spain. Though I don't remember the details, I know that there are at least two different pre-paid cards, and one of the plans was definitely more advantageous for someone who was going to be using it all over Spain making short calls for accommodation and the like. So I would ask the salesperson about which card to buy. We have to get a new number every year, because it lapses after 6 months of no usage.
Laurie