locked to kddi japan, but can use at&t usa network
 

how come when i check iphone 6s (via paid server, gsx full) it said the phone was locked to kddi japan.
but inside the phone was at&t sim card (working fine) and there's at&t apps inside the phone.
anyone have idea what's going on ???

Maybe was unlocked win worldwide unlock when it was available, it is shows: SIM-Lock: Locked?

Yes it said that locked to kddi japan, but owner can use at&t sim.
When i put other sim, it asked for actvation, connect wifi then showed : sim not supported.