Most unlocked phones can be used with almost any cell phone plan, but there are the occasional few that will only work with certain plans. Unlocked just means it was unlocked from a specific network so that it can be used with other networks. For example if you buy a phone locked to Cingular, you can get an unlock code so that the phone can also be used with T-Mobile, Verizon, etc. But you have to be careful with using an unlocked phone for a plan it wasn’t originally made for, because you might not be able to use certain network features such as text messaging or internet browsing. I have a Cingular plan, & at one point I bought a Blackberry that wasn’t a model Cingular ever carried. I could make/receive calls, but could not use text messaging or internet.