Using phone overseas

Not until it’s paid off. 

Your phone is locked to ATT. It can’t be unlocked until it is paid off. Sure, you can add an esim but any esim from a different carrier won’t work until the phone is unlocked.

Any official response from AT&T will be your phone can't be unlocked until it's paid off, but you can use international day pass.   International day pass charges you only when you use your phone connected to a foreign network. Phone usage is make or receive a phone call, send a text message, in coming text is free, or use data, even if that's only data in the background

  IDP charges  is capped at 10 charges per bill cycle equal to $100 per bill cycle