SIM

I have just gotten my brothers old phone, and I cut down the sim to fit in it. After popping the sim in, I'm now being asked for a "SIM network unlock PIN". Who do I need to contact to get this pin?

Hi I recently purchased a Xperia M Dual Sim model. Unfortunately I did not know that it was a micro SIM. Now I already have a Nokia Lumia 520 with Micro SIM. The one in Lumia was cut locally by mobile salesaman. The dimensions of the micro SIM was not exactly as per specifications with it...