OK... there are 2 Digital phone systems used here:
CDMA (Code Division Multiple Access) wich in different stages of development, is used in this country by VERIZON, CENTENNIAL and TRICOM.
And GSM (Global System for Mobile) wich is used by Orange, and it's an older technology, but more developed.
If your phone uses a "SIM" chip, it's a GSM phone, if not it's a CDMA phone... either one can be used here.
Both Verizon and Centennial offer 3G technology... Verizon's much better overall... you can connect to the internet via mobile... even DSL.
There is also nation-wide ANALOG network from the old days... but pretty handy when you are somewhere deep withing the island, using a dual band phone.