If you don't have Unlock Code yet, you can try:

http://stores.shop.ebay.com/Primo-Mobile-Unlock__W0QQ_armrsZ1

How to Enter Unlock Codes on Sony Ericsson Cell Phones (all models):

If your phone has a "JOGDIAL" (for example SonyEricsson K700i), then you must press the "down arrow" instead of the "left arrow" for step 3 & 5. By "Down Arrow" I mean directing the Jogdial down!

For the unlocking of a locked  Sony Ericsson no sim card is required.
Turn on the handset.  Display will indicate “Insert SIM.”
Press the left Arrow (<) button.
Press the star (*) button twice.
Press the left Arrow (<) button again.  
You will briefly see one of the following messages listed on devices display: “Personalized with MNC2” or “Personalized with MNC3”.  
Wait approximately three seconds.
The display will indicate “Network” and a picture of a closed lock.
Press Select.
The display will indicate “Unlock (Net) 5” and “NCK:”
Enter the 16 digit NLCK4U code (unlock code we provided you) and press okay.
The handset will display “Network unlocked” and the display “Insert SIM” appears.
The handset is successfully unlocked.