I'm certainly no expert, but do you know why your old phone supposedly won't work? I bought a cell phone in Spain more than ten years ago, and every year I just buy a new sim card for it and I can call or text just fine (though it's old style texting, very cumbersome). Maybe the difference is that I have an old Spanish phone and you have an old US phone, but it might be worth pursuing a bit further. The other option is to just buy a cheap Spanish cell phone when you're there, you can get them very cheap in connection with a purchase of a SIM card.
What carrier is your current phone on? Verizon, AT&T, Sprint? You may be able to use what you already own.
For coverage in rural areas, get a Spanish SIM card that is on the Movistar network.
http://prepaid-data-sim-card.wikia.com/wiki/Spain
is everywhere in Spain! I've gotten on in bars in villages of 30 people. I don't think you'll have a problem. Buen camino, Laurie