Let me put it this way: if some frequencies only work with some network operators, that is effectively another form of phone locking. If you want to switch to a rival network operator but they don't use your phone's 3G frequency, then you cannot use the rival's 3G network.

In Europe the 3G frequencies are standard across most network operators so you can switch network operators without having to change your 3G phone. That means any SIM card will work in the phone because the phone's frequencies work with any network operator.