I liked in Madrid for ~4 months and used a prepaid cell phone and here is what I remember from 2.5 years ago. (Note some info may have changed).

I used Amena for reasons that I do not remember and never had a problem getting service any where in the country. The exception might have been on the Metro in Madrid, but it has been too long. I know Amena and most of the other cell phone carriers allow you to roam throughout the rest of the EU/UK if not all of Europe for pricey roaming charges so if you are planning weekend trips you will be covered, but for a price.

At the time I was there Amena had a deal where you could pay a few euro (maybe 2 or 3) and set up a few favorite numbers where the cost came down to a few cents per minute for the calls to this number. I set up one of my numbers to be an international prepaid calling card so I could call the U.S. for maybe 5 cents/minute.

IIRC Amena does offer customer service in English (not sure what your Spanish level is) and I believe (but could be wrong) that most of the other carriers do also. Probably the best way to go about getting the phone is to go to a large department or discount store in Madrid where you can compare all the plans side by side and then pick the one that works best for you. I think I paid about 70 euro for the phone and a maybe a 10 or 20 euro talk credit.

Hope all this helps.