The only safest way to unlock a mobile phone is to unlock using unlock code. The unlock code for a mobile phone can be obtained either from the respective network provider or from any trusted vendor online.
So, Try getting the unlock code from your respective network provider. If you are not able to get the unlock code from them, You can try from any online services. But a credit card is definitely needed for an online transaction. So you can get the help of any of your friends who has a credit card.
You can get the unlock code for your Samsung Z400V from http://www.simpleunlocking.com/samsung-z400v/rs16wp9/
at low cost with easy instructions on unlocking process. This site is a trustworthy source to rely on accurate unlocking solutions. All the best mate.