Sim locked for iPhone

I didn't purchase my iPhone through any carrier, so paid full price for it to use with any carrier. Now, I'm traveling to another country and notice that the

"Carrier SIM" says "SIM locked". I contacted my carrier in the Canada to get the SIM PIN to unlock my phone but they told me to contact the sender. I'll contact the sender later but just want to pop up here and look for some answers. Does any of you know have any similar trouble? Or now how to solve it? Or any advice?