Not sure what programs are offered on SXM. You can rent cell phones or buy a cheap one at either of the Island's mobile providers or authorized distributors:
LIME has a "phone in a box" that is around $50 US and comes with pre-paid minutes and is popular with tourists. You can add minutes at stores all around the island.
Thanks everyone for your input. I will pass on the info to our friends who are going to the Caymans in June and need to keep in close contact with their family.