All Korea USIM(Gold, Blue+, Purple)work with only unlocked phones. 

Korea eSIM(Red, Sky) works when your phone is unlocked and eSIM compatible.

Important Notes : eSIM is not refund-able after your purchase basically. Please make sure if your phone works with eSIM.

1. How to check your device unlock status

 1) iPhone

 a) It is unlocked if you can see No SIM restrictions in Setting > General > About > Carrier Lock

 b) If your iPhone is locked, please contact your mobile carrier. 

     ref. Apple support: https://support.apple.com/en-us/HT201328

 2) Android phone

a) The chances are that your phone is unlocked if several carriers appear in Setting > Connections > Mobile Networks > Selecting roaming network > Select automatically (off) 

* Android device's menu may vary depending on brand and model.

b) To make sure, check IMEI of your phone(enter *#06# on dial), and please contact your mobile carrier.

c) If your phone is locked, please contact your mobile carrier. 

2. How to check your phone supports eSIM

1) iPhone : Your phone supports eSIM if eSIM QR code scanner opens in Setting > Celluar > Add eSIM

2) Android : Your phone supports eSIM if eSIM QR code scanner opens in Setting > Connections > SIM card manager > Add mobile plan

 * Android device's menu may vary depending on brand and model.