An NCK code is used to allow some types of cellular phones to connect to any phone network. It is also known as the "network code key" or "network unlock key."
When a cellular phone is provided or manufactured by a company that also provides cellular service, they generally restrict the phone so that it only works on their own network. Entering an NCK code is a means of removing that restriction on some types of phones. Customers may sometimes obtain the NCK code directly from the phone manufacturer if they have a valid need to unlock the phone, such as traveling to another country. Some NCK codes can also be deduced from an algorithm by using the phone's unique IMEI number.