Do you have carrier locked phone or an unlocked one ? If you want to switch numbers, you need to unlock your mobile to do so . Check whether your phone contains sim lock or not . If your phone is locked with any carrier, use unlock code to unlock it . You can get it from any of the online vendors like MobileUnlockSolutions.com to unlock it . After unlocking your mobile, you can go for any other GSM networks internationally . After that,contact your service provider and request a PAC code, once they issue it, take it to your new provider and they can port the number over.