Verified answer

Phones are not locked to a contract. Any phone can be used with pre-paid, but if you are on a contract, your sim card for the phone will be linked to that contract and that sim card will not be useable for pre-paid billing until you are finished with the contract.

Your phone can be unlocked to use other carriers. For example, if your HTC Evo was from AT&T and you wanted to use Bell, you could unlock your phone to do so.