I can’t get the PUK code anywhere

I want to unlock my SIM PIN, but I didn't set the number so I need the PUK code. My iPhone is SIM-free.

I asked my carrier today to do so, but they said this problem is for Apple. So I went to Apple Store and asked the clerk and they told me to ask in a message with a receipt. I tried to send this by message, but first of all, I can’t send it without a phone number, same as calling. Also, I don’t have a landline in my house. That’s why I asking here.

Would somebody help me, please?