I am going on international travel and planning to take my iphone (locked to At&t) with me. I am expecting some important calls so would like to keep my current number active. Now, when someone from US calls me and if I don't answer and it goes to voicemail, will I be charged for roaming? I will turn off the data, just the phone connection. I can call my voicemail number from a local phone to check voicemails and call back.