There's a very good chance your phone or smartphone
arrived locked to a particular phone network (e.g. Vodafone, Orange). This
means that if, at any later stage, you want to use it with another network, it
has to be 'unlocked'.
Now, this unlocking can be done by your initial 'locked'
operator, but they'll usually make a significant charge for the service, often
because they got you the phone in the first place at a discount, knowing that
you'd be paying them to use their network - and now you're off to a different
network, so they have to get their subsidy back.
If your network's unlocking charges seem too high, try one
of the Internet-based services advertised on this page - I've unlocked several
phones in the last couple of years, to free up how I use them, and each time
it's cost less than $10. All your network operators are doing is using a small
program to calculate the unlocking code, the services advertised here use the
phone unlocking unlock orange