How can I use my US-iPhone in South Korea?
Before buying a CDMA SIM in Korea, check that your iPhone 4s is
- a CDMA phone, and
- unlocked
Calling stateside from South Korea using a Korea-issued SIM card may be expensive. You should consider getting iOS apps (e.g. Skype, WhatsApp) that relies on WiFi or data plan to communicate.
Call AT&T and have them unlock your phone. You're eligible for this process. Check out this link for more info. You qualify for the normal unlocking procedure, but you may also qualify for the military phone unlock as well. Double up for double power!
Buy a Koren Microsim Card of your choice from a cell service provider.
Use Skype, Facetime, Vyber, Whatsapp, or any of the other apps available on the market to have free international calling/text, as international calls with most providers are prohibitively expensive.
If you have a good data connection, you can send free iMessages to friends who own Apple products internationally.