Your cell phone won't work on the crossing even with an unlocked GSM phone and a SIM. The cruise line has to have a special setup to allow cell phone usage.
RCCL and other cruise lines have it -
DCL does not. Your TMobile contact should have known that.
For DCL, only satellite phones will work. These are phones that go directly to the satellite. You would definitely know if you had a satellite phone. These phones work anywhere in the world. They are very, very, very expensive.
To figure out whether a SIM card will work in the Canary Islands, go to the carrier's website and check out their coverage map. Spain has at least two major carriers - so pick the one that works the best for your trip. The card corresponds to the carrier. I haven't been to the website you posted, but other websites have links to the carrier's website so you can see their coverage map before purchasing the prepaid card.
Back to what TMobile said - they were quoting the cruise ship price IF you were on a cruise ship that offers cell phone coverage. You aren't. Your rates will be the $.99/minute rate since you'll be using land-based carriers, not satellite. If you look at the coverage map, you'll see it extends a bit into the ocean. As long as the carrier has coverage wherever we are, you'll be able to use your phone.
For staying in touch with family, I'd recommend email. The ship will have satellite coverage for the internet, so you'll be able to access your email throughout the trip. You can always designate a friend at home to send emails to you if there is a crisis with a "non-internet" family member. It's not quite as immediate as a cell phone - but quite frankly, if you're in the middle of the Atlantic - what are you going to be able to do?
Sue