See: »
magicjack.com/faq/ The info IS in there.
During registration, you can retain your old #, or get a new #.
There IS a caveat: you cannot transfer the purchased time you had with the old MJ. Basically a new account. I believe you can still use your old MJ with the "shared" # until the time runs out. MJ gets away with this by stating they are two totally different products. Maybe someone else can chime in here to verify.
The MJ Plus has to be plugged into the computer for the registration process. Once completely registered, it no longer requires the computer. Just plug it into the power wart, the RJ-11 cable into your router, and your phone into the MJ.