See www.BocaChicaBeach.net/staying_in_touch.html for the best calling card rates (as low as 2 cents/min. to the DR with connect fee).
Whether or not a card works also from the DR depends on the specific card. Keep in mind that calling from the DR will in general run at least double the cost of initiating the call from the USA or Canada. If you plan on making lots of calls from the DR to internation destinations, it pays to subscribe to a callback service.
When you sign up, you tell them what the call back number will be. You call an international number from the phone that has the "callback" number and let the phone ring once. After you hang up, you will receive a call at the "callback" number within a minute. When you answer, there will be a dial tone, from which you can then place your call at low rates and no charge on your phone bill.
Telegroup offers this at about US$0.29/min from the DR to the USA and US$0.32/min from the DR to Canada, billed in 6 second increments, 30 second minimum, no connect fee. See www.telegroup.com for info in subscribing to a callback service.