Are you getting Sim card not accepting error message? If it is so then your Sony Ericsson W550i is locked to a particular network. To fix this you have to unlock it. Use unlock code to unlock your mobile from its network lock. To get the code you need to submit network provider, country and IMEI number of the mobile. Make sure yourself whether you have knew the network provider to which your mobile is locked, country to which the locked network belongs to and IMEI number of your mobile. If not you can find the IMEI number in two ways:
1)Go to Options>status
2)Find IMEI on the back of your mobile, behind the battery.
3)Type *#06# and press call button. It will display your IMEI number.
You can find the network provider to which your mobile is currently locked by switch off your mobile and switch on it again. A logo of the locked network will get displayed on your mobile screen. After knew all these get the code from the site Mobileunlocksolutions.com
,by submitting these to them. Unlock it yourself using unlocking instructions which is given in the site itself.