first of all, do you have the supported(which was locked to) simcard? if not, there isn't a free way to unlock your phone, but if you do, you can try these
Flash K800 CID53 with K810 software CID53 (for begginers) - Top Sony Forums but use files only for K770i and
Patching CID53 (DB2020)DB2010,2012 - Top Sony Forums to patch unlock using simox simlock generator v2.2