You can get free roaming with Three (UK) as they've just added Spain to the 'feel at home' roaming offer. This is probably the most cost effective method of using your phone whilst on the Camino if you're from the UK. To be eligible you need to be either on contract or of had a Three PAYG Sim...
This site is run by Ivar at in Santiago de Compostela.
This site participates in the Amazon Affiliate program, designed to provide a means for Ivar to earn fees by linking to Amazon