How can I stop recieving calls? I'm heading out internationally and do not want to drum up a large phone bill by taking calls while I'm away, yet I would like to have the phone to use over WiFi - FaceTime, iMessage etc.

Is there a way to turn off the phone function while keeping the ability to receive SMS messages?
Set call forwarding to forward calls directly to your voice mail or your home number (assuming you have an answering machine). As long as you do this before leaving the country it should not result in huge roaming bills. You will still get SMS though, and data roaming may cost a lot as well.