Perhaps you haven't heard of Hits mobile but you have probably heard of Vodafone right? Hits mobile specialise in providing cost effective mobile phone calls especially for Expats, and natives alike. They use Vodafone's network so you can be assured that coverage is great.
Why not purchase a mobile phone + SIM from €30,00 with €5,00 of credit thrown in. Simple to use handsets all the way to the more sophisticated SmartPhones are available. See the attached catalogue.
I've researched a few other mobile packages but Hits came out on top. You can now have a Spanish mobile phone number for just € 5 with free € 5 credit! All you need is an unlocked mobile phone (we can advise you on this, and in some circumstances may be able to unlock your phone for you). If you already have a Spanish mobile phone, then, that's no problem either to change and keep your existing number. What are you waiting for?
Topup is easy - online; you can use paypal (no Spanish credit/debit card needed), via. All Tech Plus, as well as many local shops too.
Note: pay monthly plans are available too.