I think you'll find an iPhone won't work - they've put restrictive software into it to tie it to one provider. Most other phones should work fine with a BTL Digicell sim card. I'd recommend doing it that way as you'll then have a constant phone number whenever you visit.
Remember that BTL don't interpret money credits to a pre-paid phone the same way that most other companies do. Here a credit is valid for a specified period, typically three months for larger credits, and on the date any money you haven't used is confiscated, however much it might be. The more the better from their point of view.
In the UK and so far as I know the US that isn't the practice. There the balance of your credit is good for a specified time after your last use of the phone. "Use" means making a call, sending a message, or crediting more funds. In the UK you don't pay to receive a call (by law) and I'm not sure whether receiving a call counts as "use". I imagine that in the US, where you pay to receive calls as well as to make them (extraordinary business practice, a spam caller's charter) receiving a call would count as "use".
After a lengthy period of non-use, I believe substantially longer than the time referred to above, your number will lapse and you sim card will become a piece of useless plastic. I don't know how long that takes.