Is unlocking an iPhone illegal?

My family and I are currently using At&t, but the data plans are getting expensive. We have 3 iPhones and all of our contracts would end in one year. We're planning to switch service providers after the contract end, but we still want to use our iPhones. Is it illegal to unlock the iPhones after the contract ends in order to use them at another cheaper service provider?

Thank you

Added (1). I was looking at T-mobile. On the Apple website, you could buy a factory unlocked iPhone for way more because T-mobile doesn't have a contract. It sucks. My bill at at&t has reached 240 a month. The consumers always lose.

No.

It does void the warranty and terms of service from Apple though, which means they are no longer obligated to service it or to provide software updates for your phone. It basically voids the contract with Apple that you entered into by purchasing the phone.

But jailbreaking is not a criminal act or anything like that. And Apple can't sue you or press charges. It only voids the warranty and terms of service.

Does the other service provider not carry the iPhone? Usually if your contract ends and you switch services you can get a smartphone, such as the iPhone, for cheap or free. If you tell them you are coming from another wireless service provider, they may even sweeten the deal more for you.

Unlocking an iPhone is illegal if the iPhone was bought after January 26th, 2013 and you unlock it through 3rd party means. A carrier can, if they choose, legally unlock the iPhone for you (regardless of when it is bought).

Example:
If you have an iPhone purchased before January 26th, 2013 on AT&T and your contract comes to an end, you can ask AT&T to unlock it for you so that you can use it on another network. What you can't do is perform the unlock yourself, or pay a third-party to do it for you.