You can use your own mobile phone in Brazil with the “International” option. Ask your operator about the conditions of use of your mobile phone abroad
Namely: a triband or quadriband phone is sometimes necessary. To make sure that your device is compatible with your destination, check with your operator.
Enable the “international” option: for recent subscribers, it is usually enabled by default.
Rates: they are specific to each operator and vary according to the country (the globe is divided into several tariff zones). Remember that internationally you are charged for both outgoing and incoming calls.
Buy a SIM/chip card on the spot: if you stay in Brazil long enough, this may be a good option. However, you must first register and have a CPF number (Cadastro de Pessoas Físicas), which is a little complicated…
You must apply at Banco do Brasil or at a post office (Correios), present your passport, fill in various forms, pay a tax of about 6 Rls and go to a Receita Federal to finally be issued your CPF.
It is also possible to apply in your country of origin before leaving, via the Brazilian Consulate General.
Information and forms (in Portuguese and English) on idg.receita.fazenda.gov.br
Otherwise, a simpler scheme: if you have a Brazilian friend, ask him for his CPF number.
Then all that remains is to buy a SIM card - cartão chip in Brazil - (for about R$10-25, prices vary a little) at a Tim or Claró kiosk or mobile phone store, for example, and follow the instructions. Refills are available in mini-markets, kiosks or even pharmacies.