For occasional calling to the DR, I have used a company called Acculinq ,
AccuLinQ - Phone Cards | Calling Cards | International Calling Cards, for when I need to call the DR from the US. You set up an account with them that is billed to a credit card. Then they bill by the minute. You call using an access number and a pin. It isn't as cheap as some calling cards, but there is no card to purchase or expire and it is convenient to use from almost any phone. They charge a small fee for any month that you actually use their service and then the per minute rate based upon where your are calling to and from. For example:
"USA - 48 States DOMINICAN REPUBLIC What they call non-select access is $0.239 from areas that aren't on their preferred list to call from and select access is $0.199 minute
*PREM rate: Cell phone or Remote/Special Area (If you know the number you are calling, please call Customer Service for exact rate: 800.699.9535)
*All rates are subject to change"