E911 by Subnet/IP Address

Asterisk has the capability of detecting the local IP address of the telephone.  Right now it can be exposed using a custom dial plan but that can be cumbersome to write and maintain.  From a E911 perspective, when users move around, there are not many options other than using emergency phones at a location or not allowing these "hot desk" users to call 911 from their extension.  By specifying the IP address range or ranges of the phones, you could set a DID for emergency calling based on network.  This way all users could have E911 but in a dynamic fashion.  When they move around, the DIDs would simply adjust.  If I were to go to Texas one week, then California the next, 911 would continue to work with a local DID assigned to the subnet(s) and I could simply move around as needed.  Other big players in the corporate space such as Cisco & Avaya have this capability today.  There are large organizations (mine is 1,800 admin staff) that could use FreePBX and drop these proprietary systems (and potentially save $100,000+ per year if not more) if they had better 911 capabilities.  In fact it was suggested a few times.  Compared to our Avaya setup, this is the only feature that is missing that we cannot do without as users constantly move around.  Subnet detection allows the emergency calls to go through and cut down on sending authorities to the wrong place.  I often recommend FreePBX to my customers (outside of my day job) and have gotten around it by using emergency phones placed around the building.  Even they have requested something a bit more robust.  Other options would be using the ELIN capability in LLDP which could tell the phone what Emergency DID to use but the phone would first need to support it (would be great if Sangoma models did which would put them with the big dogs in the game) then notify the PBX on what the DID is.