on youtube search for the channel GeeksBoard.
I have unlocked both zong devices, the wingle e8372H and the one with battery e5573s-320. Both worked fine on a jazz postpaid sim, e5573 I am still using the jazz postpaid sim.
Gonna switch over to jazz mbb sim soon as zong is terrible in evenings, at least I dont have to buy a new device.
edit: with the wingle e8372h I did have the lastest firmware. So though geeksboard method it downgraded to a previous firmware. Worked fine, only annoying thing is that every now and then it asks you to upgrade firmware. If you do then device locks again (but you can unlock again easily).