any help with unlocking a SE z520i?

Hi guys, I have an Sony Ericsson z520i and when I insert a different sim card i get :
Insert Correct SIM card.
Any suggestions?
Thank you!

 

Call your network to see if they will provide you with a code, if not take the phone to a shop to be unlocked directly with hardware.

 

I will try this. Thanks anyway.

 

Anytime.