When you start from SJPDP, you're only walking in France for five miles and then you are in Spain, wait till you get to Pamplona to buy a SIM card.
Don't waste your money on a natgeo SIM unless you are going through several countries.
Lebara has some great deals and their site is in english.
http://www.lebara.es
I believe you can buy single-country SIMs from NatGeo at a cost that's more competitive than the multi-country cards which most travelers buy. The multi-country cards issue a UK number, which doesn't give you the best rates in other countries. We used a similar plan from
http://www.telestial.com/ on a previous multi-country trip in Europe, and while that plan was cheaper than the NatGeo plan, it was still more than buying a single-country SIM from any of the major carriers in Spain.
At the time we did our Camino (2013), I had an older CDMA-only Verizon smartphone, which was pretty much useless in Europe except as a wifi device. I also had the older unlocked GSM dumb phone from our previous time in Europe, so I purchased an Orange Mundo SIM for it from
http://www.simsforspain.com before I left. Calls to the US and Canada were .01€ per minute plus a .19€ connection fee per call. Text messages were more than some other plans -- I believe .71€ each. Since it was a dumb phone, I did not buy data, but just used wifi for email, etc.
For the original question, be sure you have an unlocked,
4-band GSM phone -- some of the older North American GSM phones don't have the European frequency bands and won't work in Spain or elsewhere in Europe.
Regarding coverage, there was only one remote location (don't remember where it was) that my phone with the Orange Mundo SIM card did not have reception on the
Camino Frances. On the other hand, my wife just added global roaming to her Verizon 4G smartphone, and she only had coverage in the major towns and cities. FWIW, I believe Verizon partners with Vodafone in Europe, and the agreement does not provide full access to the complete Vodafone network, which explains lack of signal outside metro areas.
Good luck to the original poster on deciding which works best for you. It all sounds pretty complicated for the casual user, but once you understand all the terminology that's being tossed around here, it's really pretty simple -- assuming you have a phone which is or can be unlocked and is compatible with the European GSM frequencies.