Unless its an iPhone then it would be no.
You will have to first get the phone unlocked.
iPhone
Unlocked smartphones can be bought at phone stores, where, besides selling phone plans they also offer the possibility of buying unlocked smartphones such as iPhone or BlackBerry.
AT&T/Verizon allows you to use an unlocked phone. Also there are many models of the Apple IPhone. AT&T is less expensive than most of the unlocked phones. The company pays part of the cost.
The IPhone 4S is about $200 when you sign up with a 2-year agreement with AT&T or Verizon. If you buy an unlocked phone, it is usually $600 for each phone.
The Iphone is nationally supported at AT&T and Verizon. Although it is not recognized by cell phone companys you can have your Iphone unlocked so it will work on any GSM network globally.
Yes it is now possible to unlock all versions of iPhone 3G and 3GS
When you say "unlocked phones" I am assuming you are talking about phones that have been hacked into. The Apple iPhone can be unlocked using an App called Handbreak. It lets you modify your phone beyond what Apple allows. If you unlock your phone, there is a good chance that the warranty on your phone will be void. I do not recommend to unlock it.
The iphone will be restored using the last backup so your info will be saved and it will still be unlocked.
You can visit apple.com or visit a local apple store for unlocked iPhone.
in the UK the iphone 4 unlocked 16gb is 499 british pounds and the 32gb unlocked is 599 british pounds in the US the iphone 4 locked by AT&T is $199 and the 32gb locked by AT&T is $299. The US prices indicate a contract phone price, that is not the same as an unlocked phone. The iPhone 3GS 8GB is $474, the iPhone 4 16GB is $649 and the iPhone 4 32GB is $749 with no contract. :)
what is an unlocked cell phone