You can buy a prepaid SIM card (~ 5 euro) in the airport or in any public phone in the center of the city. You need only a copy of the first page of your passport. With this card, you can make all your communications inside Tunisia and for all other countries. For the communication price and the rooming information, please see the Website of the companies.
The current exchange rates of foreign currencies quoted in Tunisian dinar is available on the official Website of the central bank of Tunisia: http://www.bct.gov.tn . You can make exchange at banks in the airport or in the center of the city.
Many of Tunisian shops do not accept the international credit cards (Visa, MasterCard, American express…). These cards are accepted only in the majority of the hotels and in Tunisian banks.