I recently studied abroad in Spain for 6 months. It was cheaper for me to buy a prepaid phone (móvil de prepago) once in Spain than to rent a phone before I left. I bought my prepaid phone at an Orange store for around 39 euros. I then put money on it as I needed it (calls cost me 18 euro cents per minute). I don't know what you'd pay to rent a phone through that company, but if you travel frequently to spain it may be more economical to buy your own phone and add money (saldo) as you need it. I still have my phone and will use it when I go back to Spain in December.