I am looking into traveling to Puerto Rico or the US Virgin Islands in the very near future since both of those are US territories and do not require any passports. I know that I can call TO Puerto Rico for example, however I am very concerned as to whether I will actually have service while I would be there IN Puerto Rico.
I traveled to PR twice and did not have phone service. You should be able to use your phone via wifi or someone's Hotspot but that's about it. The first time I purchased a sim from Walmart. The second time I took one with me.