I used an old iPhone3 last year.
I learned if you'd had the phone more than 2 years, they were required by law to unlock it.
So I had it unlocked.
I purchased a SIM from Vodafone and it worked GREAT!
If your phone has wi-fi and you just want to make emails, not calls, you don't even have to have that.
You can pick up free wi-fi in nearly every village along the
Camino Frances.
I haven't tried this phone and sim on the VDLP, but will be taking it next month.. so we'll see.
I actually hate taking electronics, but since I'll be walking alone and am not familiar with this route, I'll carry the iPhone just in case of emergency.