If you have google voice setup, simply go into the PHONE settings on the iphone and forward to you google voice number.

your texts will still come in fine (and you could sign up for a 50 int'l text plan for 10$ if you want - or else it's .50$ a text.

If you are signing up for the int'l data (which has gotten pretty cheap and reasonable) then use google voice to text people (the official GV app doesn't have any ads like some other textfree apps do) and you'll get notifications of your voicemails directly on your phone for free.

I've used this for travel in south american and europe. The best thing about it is the costs are PREDICTABLE and fairly cheap at this point.