Just thinking, each phone has a unique mac address, and you specify this in the ephone configuration. this makes sure that even if they register, the buttons don't get a line (and therefore dial tone).
Your idea would also work, but you also need to make sure that the mac address and IP address are tied together, so you are sure you are registering the right phone (any phone is free to pickup any address, unless you do this)
Ndungu