We called our carrier a week before we left. We had a list of ports and just went down the list.
They were able to tell us whether service was available in each port and whether or not the calls would be included in our plan.
The information can vary from carrier to carrier and even vary between different plans within a given carrier. The information is changing all the time, so calling the carrier gets you the best information.
Also, be aware that sometimes new service requires updates to the phones in order to work. When we called, we learned that there was new service on one of the islands but it required an update to the software on the phone, they were able to perform the upgrade over the phone. Our phones worked on the island, but my sister who had the same carrier, but had not updated her phone, had no service. So this is yet another difference, if you have an older phone with a carrier, you may not get service, while someone with a newer or updated phone from the same carrier would get service.
If using your cell phone on the trip is important, I really recommend calling your carrier a week or two before traveling and getting the latest and greatest info.