Phone plan

If you have an unlocked GSM three-band phone or smartphone, you can buy a SIM chip in Spain that will be the most economical for phone calls, SMS, and data in Spain. Calls outside of Spain can be expensive.

@Jme I have used several and they were all fine. It is a question of which gives the best coverage, and for me the cheapest for data. The one I used for the last couple of Caminos was a pre-paid Orange. I went to the big department store (El Corte Inglés) where they speak English and they set it up for me but there are Orange outlets everywhere and many would have someone who speaks English. It is very easy to recharge in any supermarket, tobacco store, etc. The only problem is Orange did keep sending notifications (of "special deals") in Spanish, which I just ignored.

For me Orange.

25 Euro for my Camino for 5 euro talk time and 2 GB data for 1 month. (2015)
15 Euro for my Camino for 0 euro talk time and 2 GB data for 1 month. (2016). *I got the 5 euro card and added 10 Euro value, then use the full 15 euro value to buy 2 GB data.

Everything was done by the shop so I don't have to worry about the code, etc. I only use data. For the few times for voice call I use Whatsapp for data call, text, pictures, etc.

2 GB data should be ok for up to a month unless one is a big data user.

There are other options like Vodaphone, Movistar, etc if you can find their mobile shop. I find it easiest to just go to the smaller 3rd party mobile shop either at the train station or bus depot (usually where you will need to go before your starting point of your Camino). Those shops tends to favor Orange. You can also tell them what you need (more voice or data), more flexibilty.

On the Camino now and using Vodaphone. Got it in Pamplona for EUR15 for a month with 2GB data. So far so good. They gave me the sim for free.