I have a Motorola Rzr V3 quad band that has been unlocked. Miss Ginny uses it with a T Mobile sim card for everyday use. I can put the Digicel sim in it and see both the T Mobile and Digicel networks and can make calls but at the Digicel roaming rates which are considerably more than the rates in Jamaica.
However, I can call *120# to check my credit balance at no cost which also keeps the sim active
If I use the Digicel Coral 180 I got with the sim in Jamaica it gives an error that the network can't be found. So the upshot is that yes you can use the Digicel sim but only in an unlocked GSM 900/1800 phone. I guess you can get the code to unlock the Digicel phone from Jamaica but I don't think it's worth the effort.