- With a payment plan (typically 1 year or longer) where you get a mobile at a reduced price because the phone operator gives money back to the airtime provider. Typically this mobile phone will be locked to your selected carrier
- Buy just the phone only, unlocked and without any carrier. Then you can goto any carrier and ask to join, or just use prepaid SIM cards. The phone is naturally more expensive. In fact 600 GBP from Amazon UK
It is therefore a tradeoff. You can have the freedom of the unlocked phone without any ties but it costs more.
I am not totally sure what sort of dim-witted system exists in America, but lately many people have found out that their (by design closed platform) iPhone does not react too well to firmware updates when they have expressely unlocked the phone either for anySIM calls or applications.
Here it is in pictures