If you bought your phone from Cingular at a low price because you were becoming a client or, as a client, upgrading to a more expensive phone, it is very likely locked by Cingular. If you bought it elsewhere and have been using a Cingular SIM card on a pay-as-you-go basis, it very likely is not locked.
To use the phone in the DR and pay local rather than roaming prices, you will need to get an Orange SIM card and buy time cards. If your phone is not locked, just take out the Cingular SIM card and put in the Orange SIM card. you will then be using the number Orange gave you and paying for your calls with an Orange time card. If your phone is locked, you will have to get it unlocked first. That can be done here.
When you return to the US, take out your Orange SIM card, put in the Cingular SIM card, and resume using your Cingular number.