My sim is still under someone elses name

Hello. So I've recently moved to Japan and I got my JP sim card from GTN Mobile. Its working although I'm yet to pay my first bill (signed up like end of Aug). I need to buy some books on amazon but of course I need to register my phone number. When I entered it, it was declined stating that the number belongs to someones elses account.

I'm trying to thick, maybe cause the person hasn't deleted the number or because I haven't paid my first bill so it hasn't officially transferred to my name. I'm not sure but all I know is I really need to buy these books and they're not available in the bookstores I've been to.

Any advice or thoughts on why this is and what to do?

by JYPinCebu