South America Money Exchanges and Phone Questions

I am going to South America to live in Santiago for a year. I will be vacationing in Columbia for 3 days, then to Buenos Aires for a week, on my way to Chile. My question is this. All three places have different pesos. I will need money for taxi's from the airport to the places I am staying, so will need a bit of the local cash. Thereafter, I assume I can just pull local currency out of an ATM. How can I exchange just a bit of taxi money? Also when I get to Santiago, I am to give my landlady a months rent in Chilean pesos. I suppose I can find an ATM to pay her that larger amount. Also, while living in Italy, I brought my smart phone and got a SIM card there. So I did not have a phone when I got there for the first couple of days. I will need to use a phone of sorts in both Columbia and Buenos Aires, to call the landlords at the places I am staying. Any advice? Thank you!!