My advice is get a CPF and get a contract phone otherwise you will pay through the nose on pay as you go.
If you are rarely going to make calls then pay as you go will do, but as I found out to my cost, you will pay roaming charges to both make and receive calls when you go into other states. The roaming charges are not cheap.
Get skype if you can to make calls as it works very well in Brazil and you will save a fortune.
Another trick is to buy a skype-in number for the UK and buy skype-out credits. Divert your skype id to your mobile in Brazil. So if anyone wants to call you from the UK on your mobile in Brazil then they just dial your UK skype-in number and your Brazil mobile rings
.