It's definitely locked to some cell phone provider. If it were unlocked, the phone would not be saying network locked/invalid sim/asking for aa network NCK code. Being unlocked doesn't always mean compatible but even an incompatible phone wouldn't give you such an error message; it just wouldn't be able to connect to your network.
In the past, there have been phones that have been "personalized". A personalized cell phone differed from an unlocked phone by that it would only work with one other company's sim card. Because of that, you really need to try sim cards from at least 2 other providers other than the original locked carrier. I haven't seen a "personalized" phone in a while, but it's always still a possiblity, especially if it's an older model.
