It is possible to unlock the IMEI of most locked cellphones. Federal Communications Commission guidelines state that carriers are not required to unlock postpaid phones under contract or with outstanding account balances. Carriers can also legally refuse to unlock a prepaid phone until one year after its initial activation.
Unlocking a phone does not guarantee that it works across all American cellular networks. It also does not guarantee that the phone works on any international network. An unlocked phone can only be activated on networks that use compatible technology; this is a technical limitation and is not related to IMEI locks.