I'm in the same boat. Will in in SP for a week in a couple of months and am looking to have a number for local calls. I have an unlocked phone to bring with me that I could put a local SIM in, I just don't know if I'll be able to get one (no CPF).
if that isn't an option because of the lack of a CPF, how common is wifi access? I'd be ok with paying for a skype number for this too.