Re:Registering address (Score 1)

Registering an address would be a good idea for keeping tabs on if a device was moved to a different area. Most broadband connections will have a pool of IP's for a given area. If that IP range was registered with the VoIP provider, it would help detect when a person moved by checking if they are still within the same range. It would require assistance from the ISP's to provide possible ranges to the VoIP provider.

Also, to help detect -- VoIP providers could utilize the NCOA (National Change of Address) database from the post office. Since, most people have their mail forwarded.

Due to the topology of the Internet it is difficult to find out a persons physical location based on the IP address.

But, just a few random thoughts.