Insert an unacceptable SIM card to locked phone, now phone should ask unlock code (NP code?). Easy just enter code.
Not working?, alright now remove the SIM card and try this code ###825*09# . got the prompt for code? enter the code.
Failed above, try *983*8284# . It should work.
Try *983*865625# also it’s working on few models of ZTE phones like F102 (Thank you ‘Lomarko’ for your comment)
All above methods failed? Try Google search and share any new methods via comment with your phone model, let’s help each other.