I did a bunch of research on this and here is what I found out:
1 - No matter what kind of cell phone sim card you have they won't work in the middle of the ocean. No cell towers = no service.
2 - If you have a satellite phone (and the prices are enough to scare the pants off of you just to rent one) you will have phone service because it is satellite based, not cell tower based.
3 - The Canary Islands are part of Spain for a couple of services I looked up.
I actually did a cost analysis (anal retentive blood sucking bean counter) and it was actually CHEAPER to do $75 worth of internet on
DCL and then buy two $25 phone cards (total of $50) for a total of $125, than do JUST cell service or a satellite phone because of the cost of the cards/rates/phone rentals. And on the phone cards we did factor is a hit of 30 minutes everytime we used a payphone (highest rate charged) to call home. We figure we will call one kid and have them conference in the other kid and we can all talk at once. Of course we are all rather geeky so YMMV.
Laura