To expand: the problem is the VoIP client being able to *furnish* an approximation of where it is, to permit the selection of the proper Public Safety Access Point (or equivalent).
VoIP clients can't provide such information unless they KNOW this information in the first place. The only somewhat reliable way to know this information is for the hardware device containing the VoIP client to also contain a GPS system or some equivalent (cell triangulation, querying cell transmitters, triangulate RTT measurements to known IP addresses) That is a big problem.
If each end-router supplied that data, through *some* easily queriable protocol, such clients could retrieve it, and then decide (in some fashion) where to send Emergency Services Request calls (or furnish it to their carrier, if they have one, for similar purposes).
And if I am using a laptop communicating with IP over Bluetooth to a GPRS cellphone in order to establish an IPv6 tunnel to my colocated server in Germany, then which router should my VoIP client query? My home DSL router in London? The router at the colo in Germany? The GPRS cell transmitter? The Japanese IP gateway router between the cell network and the Internet? This is not a simple technical problem. There are human factors included as well, for instance, should there be a separate specification for different classes of device so that a device with a screen greater than 320 x 320 pixels should ask the user to confirm (or override their address)? A quick knee-jerk fix will only create new problems and muddy the waters further if it is presented as the ultimate solution. --Michael Dillon