"Have you ever used WhatsApp? If the caller and the receiver both have the free app on their cell phone, then calls and texts are free anywhere in the world"
Not quite true. A lot of folks have been burned with surprise cellular roaming DATA charges. WhatsApp requires a data connection, there's no magic. If you can find free WiFi, and your phone is set to use the free WiFi and not cellular data, then WhatsApp and other calling apps can make free calls worldwide, sure. It is not unique about doing that.
But you need to be aware it is still using data. You need to know, on your phone or a burner, if it is set to use Wifi or cellular data, what the preferences are. And if you think you've found "free wifi" be aware that's often a big security risk.
Whether you can use your phone down there, will depend on whether your phone has a SIM card, what frequencies it uses, or what LTE "bands" it uses....the devil is in the details. Ask your carrier what their deals are. A friend of mine is flying into Brazil, Verizon told him he's got free international calling "but" they've somehow got that entangled with data (not free) and if a call goes to voicemail...he had to go through hoops to set things so "free" would really be "free". At least, for the time he'll be in coverage areas.