streetshirts - 4:03 pm on Sep 6, 2011 (gmt 0)
Some phones (especially in Europe) are locked to your current carrier so they won't work with a different operator (i.e. you can't replace the SIM and expect it to work). This tends to be when you get the phone for free (and they want to tie you into a contract).
You can get them unlocked at markets and sometimes online so that they will work with all mobile networks.
I believe iPhones etc are all SIM free (or unlocked) when you buy them.