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Yes if you are connected to the internet. Calls and texts would be expensive.
You can just buy a sim at the airport or many stores and re-charge when needed if you need to call someone. (or use WhatsApp/skype)

Hello.
You need a cpf for the SIM card I believe. But you can still get a sim with passport info.
The sim (pay as you go). Will not work outside of Brazil but what’s app will work.   Of course you have to make the payment on the card.
A cell
Plan will work if you get the right plan.  But as Tex said it’s expensive

Vivo sims can be used abroad, if you’ve got the right pós-pago plan, etc. 

Telegram

does not require SIM card to work.
(However, I believe you need SIM card for confirmation, to install it on a new device. They send confirmation code. Need just one phone number for all your devices. So, make sure you have it installed before you travel.)

VV
Are you in Ukraine and coming to BR?
As I stated you are going to need an internet connection. You may buy a BR card with "X" MBs/minutes and recharge, or find a free Wifi somewhere.
Be sure to turn off roaming or you will have a large payment