It depends why/how your Samsung sch-r740c got locked in the first place, but in most cases you will need to root the phone.
You can find details here if you are interested in doing it manually, http://forum.xda-developers.com/showthread.php?t=2617484&page=1
Although I would recommend contacting your phone supplier or Android phone retailer if you are not experienced, as messing this up can make your phone useless.
I have been through a few Android forums mentioning a problem with the SCH-R740C Cricket Samsung Galaxy having this issue a few times, is that the model you have too?
