Some mobile operators are allowed to unlock the iPhone after the end of the contract. But the contract should not be a financial debt.
For example, on the official website of AT&T you can apply to unlock via the web form. However, online unlock iPhone services offer some of their services for free, for example [url]letsunlockiphone.service[/url]. Here the list of free unlock iphone tools:
1. FREE iCloud Blacklist Checker [Lost / Stolen] checker - if you know IMEI # you can check for lost or stolen status of iPhone. This tools will be intresting any one who want to buy used iPhone from ebay or amazon.
2. FREE Find My iPhone Status Checker - also useful service for those who want to buy an iPhone and do not get to the cheater. Any way if you have iPhone locked under icloud you can unlock it here.
3. FREE iPhone Simlock Status Check - check if iPhone is locked under carrier or not.
4. FREE iPhone AT&T Contract Check Service - use this service if you want to unlock iPhone for free. For example if contract is OFF you can unlock iPhone for free throw official AT&T web site.
5. FREE iPHONE CHECK Warranty, Serial, Model - excellent service for all who want to learn more about the iPhone.
6. FREE AT&T iPhone 6, 6plus, 5S, 5C, 5, 4S, 4 [CLEAN IMEI] - And finally to all owners of cleen iPhones (not stolen, completed contract, will not be lost, not in the blacklist operator with disabled FMI - Find my iPhone tool) FREE unlock AT&T!
More Free iPhone unlock tools you can find here http://letsunlockiphone.services/product/free-tools-unlock-iphone/

P/S if you're unlucky and you turned out to be the owner of a cellular network locked iPhone (black list, the debt under the contract) this service will help you also . For example unlock blacklist iPhone 6, 6Plus, locked under AT&T carrier can be unlocked here

akinlolusewanu9:
How to check if your iPhone and iPad is factory unlocked or not http://phonegist..com/2013/08/how-to-check-if-your-iphone-is-factory.html

You can check for free carrier name of iPhone http://letsunlockiphone.services/how-to-check-network-iphone-locked-imei-checker/, blacklist status iCloud Blacklist Checker [Lost / Stolen]. Also you can unlock any iphone model by IMEI number. Check how to unlock iPhone http://letsunlockiphone.services/unlock-iphone-how-to/