One thing to be careful about is if you receive a call that you wish to not answer, don't hit the button you would normally use to silence the ringer and send the caller to voicemail. If you do this while overseas, the phone network will consider this as an instruction and will charge you the international rate for the call for the entire length of the voicemail.

If you're overseas and don't touch any of the "reject buttons" and just let the phone ring and ring, the domestic network will push the call to voice mail and you will not be charged a cent.