We will be traveling December 1-8 (Saturday-Saturday)
Most will not unless you pay extremely high roaming charges, have an international plan, or talk to your cell phone provider in advance and upgrade your plan to an international plan temporarily.
We will be downloading an app called "whatsapp" to our smartphones. It works with wi-fi, which the resort has included. You can text and call anyone you want through this app through wi-fi as long as they also have whatsapp. Facebook messenger is also an option to use with wi-fi.
As far as tipping on the resort, US dollars are fine. Tips are included in the price of the hotel, however, the average person working in the Dominican Republic only makes an average of $100-$200 USD/month, so any extra truly goes a long way (we usually bring about $50-$100 in singles just for tipping!) However, if you are going off of the resort, not everyone will take USD, and you might need to use a credit card (I recommend checking which of your cards has the lowest foreign transaction fee!) or exchange into Dominican Pesos. You can do this by taking money out with your debit card at the ATM when you get there, or exchange in advance. If you can do so at your bank in advance, this is my best suggestion. Otherwise, the airport will usually do it (however, exchanging at the airport will cost you an average loss of about 12%).
You will need cash to play at the casino. They accept US dollars, Euro, or Dominican Pesos. You will receive US dollars if you win!