This often means making a decision before you know where you stand with other employers. If you are not ready to make a decision it's appropriate to ask for time, but usually only a week or two will be granted. If a contract is sent, be sure to retain a copy. Sign and return the original with your letter. Once you accept an offer, notify other employers with whom you have interviewed of your decision.
Phone calls are appropriate.
If you reject the offer, indicate it in writing. Be tactful, and indicate that you appreciate the offer. Try to leave behind a good impression. Mention something about the position that you like very much. Mention that you were very pleased to be offered the position.