Whatever you do, don't put a Dominican phone provider's SIM in your (I'm assuming expensive) US phone. You can go to Claro or Orange or Codetel and buy a cheap flip phone which will cost less than $20 US. This phone will come with a "pay as you go" plan and free (100 - 300) minutes. Buy a telephone card for an additional $700RD ($19 US) and you will be talking and texting for quite some time.
Outgoing calls will be about 4 cents per minute and incoming calls are free.
Put your US phone in the safe and give your contacts your Dominican number for the duration of your stay. I use an Android and a Blackberry in the States but I turn them both off before the plane to D.R. takes off. I have had the same Dominican cell phone for years because since nobody wants a cheap "flip" phone I can't give it away....and more importantly I don't have to worry about it getting stolen.
Blackberry's are hot there right now and if you pull yours out I guarrantee that someone you don't even see will be thinking of a way to get yours from you.
Please don't give incomplete or misleading information to the OP.
Cheapest prepaid calls are with Viva, 8 dominican cents per second plus taxes, costing one minute about 15 US cents. That is as cheap as the calling gets in the DR. Orange and Claro are both about 25% more expensive. Disadvantage of Viva is they do not have as good coverage as Orange or Claro.
Calling rates are same for calls anywhere within the Dominican Republic landline or mobile as well as for calls to the USA and Canada.
There is no way the OP can get a calling rate of 4 US cents per minute as you mentioned, only on corporate plans.
Viva also has cheapest phones now, they have prepaid phones for 400 pesos including free prepaid activation and 100 pesos calling credit.
For 500 pesos plus taxes per month you can get unlimited prepaid Blackberry service. It can be also purchased for 25 pesos per day, 150 pesos per week, 300 pesos for half month or 500 for full month, plus taxes.