Unlocked or Locked HTC one

[Android Question]

Hello,

I just purchase an HTC one and when I bought it, it said unlocked but I don't think it is unlocked because my SIM Card isn't working!

How do I check it before sending it back to the seller.

 

[SiempreTuna]

Easiest would be to take it to a store and have them check with a SIM they know works.