Once you've unlocked your cell phone, or succumbed to buying a new one, you have three options in cell phone service providers: Digicel; CCT Boatphone; or bmobile (Lime). I have little experience with Digicel, but HB uses CCT Boatphone and I use Lime. Lime's service seems to be better overall with fewer dropped calls and dead spots, but CCT generally has better offers and rates.
All three providers offer monthly plans. I've seen CCT advertise unlimited BVI & international calling (some restrictions apply) for $120 per month. If you're going to be calling internationally frequently, this may be a deal for you. On the other hand, the majority of people on the island use pay-as-you-go, probably for a reason.
Pay-as-you-go rates for on-island calls are about 1-2 cents per minute. On-island texts are 10 cents each (to send or receive). I don't know about data rates, as neither HB nor I have a data-enabled phone. International texts are 20 cents each, and it costs about 30 cents a minute to call the States. One major difference I've noticed between CCT and Lime is that CCT is always around 30 cents a minute, whereas with Lime, the rate goes up during times of heavy usage, and I have paid 55 cents a minute for certain calls.
So if you're going to spend more than six hours per month on the phone for international calls, a monthly service plan is probably your best bet, but I find the flexibility offered by pay-as-you-go to be preferable, as I'm not paying a set amount for the months I'm hardly using my phone.