Have you ever put a Digicel SIM card in a Sprint phone before? I thought it had to be quad-band phone on the GSM network, such as AT&T or T-Mobile, to work. Even if it is AT&T or T-Mobile, the phone still needs to be unlocked. I use an old unlocked AT&T phone with a Digicel SIM and it works great.
Both Sprint and Verizon use CDMA (Code division multiple access) technology
AT&T, T-Mobile, etc use GSM (Global System for Mobile Communications) as does Digicel and Lime