Xperia V Sim locked after update

[atingarg]

Hello friends
I have just updated My Xperia V LT25I to Android 4.3 build 9.2.a.2.5 using flashtool

After update, my phone got sim network locked and asking for unlock PIN for network unlock.

I don't know how it got locked. because it was perfectly fine before update

Can somebody guide me on how to unlock it again?

regards