Is it true that an iphone purchased from, say, AT&T will not work with another carrier?

Is it true that an iphone purchased from, say, AT&T will not work with another carrier? - 1

It's not only true for AT&T, but it's true for pretty much any carrier out there.

iPhones usually cost $600+, which not many people just have that much money to spend on a phone. So, carriers created a 2 year contract system. In this system, you pretty much sign your life away for 2 years, and get the phone at a much lower price up-front. So instead of paying $600 for the phone, you might pay $199, which is far more reasonable.

Now the contract says that you have to stay with AT&T for 2 years. If in those 2 years you find that the bill is getting too high, you can't just take the phone to another carrier as it is locked to AT&T. Also, if you don't pay the bill, your credit will be damaged.

Once the 2 year contract is up, you can then have AT&T unlock the device for you. This will allow you to use the phone on any other supported carrier around the world.

It won't until it has been unlocked from AT&T.

Yes, since it would be locked to that carrier. If unlocked, then it could work with any other.

There are carrier lock features that can prevent the phone from accepting a SIM from another carrier, yes.