Smartphone ask for NCK code

Last year I bought a smartphone online, few months ago I bought another smartphone so as I am not using my last year smartphone I send it to my brother.
He received it but he says when he insert his sim card they ask him to write NCK code.
He asked me about it but I have no idea what is that.
I googled and they say it is when you wanna unlock it you have to call and ask mobile operator for that code.
The problem is I bought that cellphone online on yahoo auction, when I received it my sim card worked just fine.
It seemed to be unlocked cellphone...

Anyone knows what to do?

The phone is indeed locked.
Normally the phone is locked to a carrier.
You do not mention the phone type/simcard you have used.

Maybe you have used the same carrier as the phone and did not got the NCK.
Your brother (overseas or in Japan) is using another carrier.

A little bit more information would be appreciated.

It was a nexus x5 I bought online.
My simcard was linemobile.
My brother dont live in Japan...