Any market will have a mobile phone stall who will unlock for you - and so will most independent retailers.
The big companies are not actually allowed to lock their SIm cards into their phones, it is unfair practice, but they do it.
When you upgrade / replace - find an independent retailer who is not annexed to any of the big providers, and choose your phone there, they will fit your SIM card into your new phone for you.
noooooooooo please dont go paying for it. i unblocked a few of my own phones myself by checking out a few sites on the internet. just google something like free samsung unlock code or similar things like that and i bet you will get it done for free on the net its worked for me everytime
good luck