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

All 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 with your IMEI(Check on your dial '*#06#') 

     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 for unlock. 

2. How to check your phone supports eSIM

   With default 'phone' app., press '*#06#'.

   If eSIM IMEI or EID number starting with '89...' can be found, it supports eSIM.