I will share my experience and it is your choice whether to go in for paid or low cost unlock service.As far as I searched, i was not able to get any legit free unlock service for the mobile model that you have specified.I just wanted to warn you that do not be specific o free and type in wrong unlock codes.Chances are there that you may run out of unlock attempts thereby your phone might get bricked.I suggest you can approach Simpleunlocking.com and specify the country,locked network of Sony Ericsson K750i. Based on the availability you can get the permanent unlock service along with free instructions.
