Facetime not available in other countries? (during vacation)

Hi,
I came to Korea for winter break and brought my iphone 4 with me.
I had my iphone under airplane mode, but connected to wifi.
I tried to call my friends in US using facetime, but everytime I tried to make a call, it rings for 1-2 times and shows "FaceTime failed" and asks me to "Call back" or "Cancel".
So, I disabled the airplane mode and tried it again, but it did not go through either.
Can anyone please assist me solving this problem???

Thank you