the only thing is that you will not have a phonebook, or contactrs when you put your BOOST sim into the older phone. you can manually put the numbers into the phone, but once the phone gets powered off, you will lose all of them. this also is the same for NEXTEL sim's, unless they originally came from that phone series. with the phone technology increasing, so do the sim cards. as newer and newer phones come out, they have newer sim's with more options on them not available before. things such as direct talk and messanging