Which company is my Japanese iPhone locked to?

Recently bought an iPhone off someone who said the iPhone was unlocked. Put my SIM card in it and will not accept. So obviously it is still locked. I have no problem unlocking it myself, but I don't know where to go to get it unlocked as I don't know where the iPhone came from in terms of its previous carrier. The iPhone is on hard reset so I can't access the settings or anything. I'm in Japan. Does anyone know how I can find this out?

Most little electrical shops can do that for you very quickly and quite cheaply

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