how to find which carrier is my phone locked to

I have a samsung S2 (GT-I9100), I was using it in my country (Tunisia) for years, then i went to belgium, and bought a sim from there, and it also was working well. When i went to back to tunisia, I tried to update my android (with the Belgian SIM on) version from 4.0.3 to 4.1.2, but the device got locked! each time it asks me for the network sim pin. How can this happen? the device was not bought from an operator. And my main question is how can i find out which operator locked my phone, so i can ask them for a PIN?

Thank you