Here's the problem. You hear, "Oh! You've got to put a Sanish SIM card in your phone!" And you do. So now, local calls are cheap. Excellent. Problem "solved". But everybody back home is trying to call you ("Have you left yet?" "How's the food?") and they get nothing because your old SIM card is buried deep in your in your backpack. So now your inbox explodes with panic emails because no one can get ahold of you (unless of course, you explained to every person who has your cell phone where you are going and why they can't contact you). So now, even though you are trying to enjoy the Camino, you are forced to diligently sit down and answer emails apologizing and giving everyone your Spanish phone number.
So now, Aunt Mary calls the new number to wish you a happy birthday. The call to you is nothing to you because you have a Spanish SIM card. But the call to her is charged at huge international rates because she doesn't have an international calling plan. And she's on a fixed income. When her bill arrives, she has a heart attack. But, all's well that ends well, because you saved money by having a Spanish SIM card.
But, if you had kept your phone number and gotten an international plan, it would have been no cost to Aunt Mary and she'd be alive today.
My only point is this. Tossing in a Spanish SIM card solves one problem but creates several others. Are you really going to take the dozens and dozens of hours to explain all of this before you go? Do you realize how many people have you cell number? Are you prepared to deal with each one before you go? Are you going to take the time, on the Camino, to explain how international calling rates effect them if you are using a Spanish SIM card?
Or, why not buy an international calling plan before you go, keep your number, and save Aunt Mary's life.