Many 800 numbers CAN be called from the DR. It all depends on the specific 800 number and how it is set up.
The Tropicard for example has a 800 toll free number that CAN be dialed toll free from the DR, as do many other calling cards. But, the local access numbers, most of which are 800 numbers, are different for each country they support. Whenever a calling card has an 800 number listed as the local access number for the Dominican Republic, it is a toll free call. If it is not listed as specifically for the DR, it probably cannot be called from the DR. This is explained at the link below.
To dial a toll-free type 800, 888, or 877 number from the Dominican Republic, dial 1-880-number. You'll then be prompted to enter the number you wish to call. At that time, you can enter the whole number (800,888,877, etc.). The call is not a toll free call in this situation.
Anybody using AT&T, MCI, Sprint, etc. must enjoy getting ripped off. There rates come out to more than US$1.00/min. and some companies like Sprint have high connect fees on top of inflated per minute rates.