The Sony Ericsson K550i is a GSM phone, which means that it uses a SIM card to communicate account information to its chosen mobile network. Replacing the SIM card in this phone is the same as switching the account it uses. Mobile providers often install a SIM subsidy lock on their phones to keep the phone from using any other SIM card than the ones for its network. Unlocking this lock requires the unique SIM unlock code to your Sony Ericsson K550i, and your mobile provider will often give it to you free.
Locate the phone's 15-digit IMEI number. Check the sticker under the battery in the battery compartment or the sales paperwork that came with the phone or dial *#06# on the phone to display it on the phone's screen.
Call the mobile-service provider that branded the phone. Explain your situation to the representative and request a free SIM unlock code. Give her the IMEI number to look up the SIM unlock code and an e-mail address to send it to.
Press the Power key until the K550i powers down. Slide the back-cover catches to the left and remove the back cover. Remove the battery and the current SIM card. Insert a new SIM card into the holder. Put the battery back in and slide the back cover back on the phone until it's secured by the catches.
Turn the phone back on. Press the "OK" button when the K550i displays the SIM-card error message. Use the keypad to enter the SIM unlock code when prompted. Press the "OK" button again.
You can tell which company installed the SIM subsidy lock because its logo is usually on the phone and in the phone's software. Only the mobile provider can tell you whether you are eligible for a free SIM unlock code for your Sony Ericsson K550i. Providers often have a set of guidelines and make decisions accordingly. The customer-service representative may give you the code immediately over the phone or by e-mail, but it may take several weeks to receive your SIM unlock code by e-mail. Ask the representative how long it will take.