Many hotels require that men wear long pants to dinner, especially at the a la carte restaurants, so make sure you pack a pair. Most buffet restaurants have relaxed the no shorts rule in the last year or so, due to the increasing numbers of North Americans traveling there. Some people dress up a bit in the evening - ie. It's basically up to you. The same applies to casinos.
Every hotel has their own procedures for long distance calls.
Some Canadian and U.S. calling cards work sometimes. There are prepaid phone cards available in a lot of hotels. Yes, you can count on there being Dominicans at the airport when you come out wanting to carry your bags, and they can be quite persistent. It is usually a very short distance from the airport exit to buses, taxis, etc. Or, if you do have heavy luggage, the easiest thing is to let someone carry them.