It's not as simple as assigning a static IP......The VoIP phone is fed off of a gateway, router or cable modem etc that may have a static IP but.....the phone itself can be moved anywhere with a broadband connection and still register to the serving VoIP switch. I can take my Cisco phone in NY and bring it to California and have it register to the serving soft switch in NY without issue. Problem is that if I make a 911 call, emergency services will respond to where I originally had my phone registered in NY.
It's up to the customer to update their geographical/ PSAP location to have emergency services respond to the correct location. VoIp providers only have to provide the customers with the ability to update their geographic location for 911 purposes. There are 3rd party providers who handle that.
Google Dash911 or VIXXI