Thanks Effendi, it looks like $1.43/minute to call the US from Brazil with Oi. Wouldn't it work to buy a calling card like this:
http://callingcards.callingmart.com/...hp/Continental
which has local access numbers in Brazil, and use it to save a lot of money? The US rate with that card is $0.036/minute. So to call the US, I would pay Oi (for example) the local calling rate, and Continental $0.036/minute for a huge savings over calling direct with Oi.