If you get it and need to unlock it, search for instructions on how to unlock it on the internet. I recommend searching BEFORE you buy, to make sure you find good instructions.
Works great as a SIP phone extension for Asterisk based open source phone systems IF you unlock the device
If your going to use it for anything other than EARTHLINK, you have to UNLOCK the device. Linksys and EARTHLINK wont help you do this because they want you to have to use it with their service