most US carriers will work on the DR as "roaming". It's pretty expensive, but uses your US number. Check with your provider before you leave to make sure it's available and ask for a discounted International Roaming plan/rate.
if you're phone's not unlocked, and most aren't unless you bought it specifically that way or had someone do it, you can get a new cheapie phone from claro (or orange I think) for around the same 350 pesos you'd pay (maybe a bit more) for a new sim with number.