The NCK number or code is provided by the Service Provider (SP). It is used to lock the cell phone to the Service Provider's SIM card.
I couldn't find any reliable and free unlocking source for a Sony Ericsson K550i from online. Most of the free offers were looking to be scam with many negative reviews. If you want to unlock your device safe, you need to spend something for it. For example, you can visit Unlock-Zone.com
and get the accurate unlock code for your Sony Ericsson K550i at affordable cost. Good luck.
1 - Switch ON your Sony Ericsson K550i without any SIM Card,
2 - Press the left arrow key/button once,
3 - Press the button * twice,
4 - Press the left arrow key/button once,
5 - The message "Personalize with MNC2" will appear, (or you will have displayed an unlock menu, select the lock you want to unlock)
6 - Enter the Unlock Code.
If your Sony Ericsson K550i does not have a left arrow button/key(e.g. Sony Ericsson W950i) and on a few phones(not all) that have a "JOGDIAL" you must press the "down arrow" key/button instead of the "left arrow" key/button for step 2 & 4. We mean by "Down Arrow" to direct the Jogdial down! In the Sony Ericsson W950i and some other models, the down arrow button/key would be located on the side of the phone.