Private locutorios (telephone offices) are less common than before, though most have internet service as well. International calls are expensive.
Local rates are low. Free SIM cards, or those with saldo (credit) charged to them are available for a small fee. Local SIM cards are much cheaper than roaming, but do not allow international calls to be made, only received.
The best cell-phone companies are Tigo, Personal and Claro; their tarjetas (cards) for charging credit to your phone are sold at every newsagent or you can charge credit over the counter via minicarga, where you see signs. Claro SIM cards can also be formatted to work in both Brazil and Argentina. Cell-phone prefixes usually start 09.