Q: Is my iPhone unlocked?

Go to Settings>General>About>Model number. What is shown there? Did you make this purchase online, or at the actual Apple Store? If the actual Apple Store, then check your receipt. It will say if it is an unlocked phone or a T-Mobile phone.

Try using a different SIM from another carrier to find out or

call Apple Care and ask.

That looks like the T-Mobile version. If it is locked to T-Mobile, you would have to ask them to unlock it. If you really want an unlocked phone, and you purchased it a week ago, you would have around 7 days left to return it to Apple, and purchase the one you actually want. That would be a lot easier than getting it unlocked by T-Mobile.