My Motorola RAZR has the Digicel sim in it. Here in Florida it shows the AT and T, T-Mobile and Digicel as available networks although any call I would make would be charged at Digicel roaming rates.
*120# will show your credit balance
*124# will show sim expiration date and credit expiration date.
No cost calls and keeps the sim active
Soon as I land at Sangster the phone works.
Another thing I do is to top up the day before I get to JA. with enough credit to get 100% bonus. The bonus credit has a time limit and cannot be given away but it is used first so that when I leave I still have all the credit I bought. As long as you top up within the expiration time the credits are cumulative.
Of course I give most of the credit to the kids from the handset once I get back to the US.