In the US, that might be true, but, I'm betting that could be very wrong in places like London. I'm betting the station where you boarded the Tube could be a LONG way from where you make the 911 call.
There are very few places in the underground tube system where you can make calls on your mobile. Outside central London where the tube runs aboveground I would expect that GPS reception would be available wherever mobile reception is available, after all the tube trains have lots of windows. But you do point out that it would be shortsighted of mobile operators to not use the location information that is already available in the cell base stations. As for VoIP, well if that is not running over GPRS or 3G then I suppose it's running over Wi-Fi and that the user has to authenticate in order for the Wi-Fi access point to accept his MAC address. Maybe we should lobby government to require Wi-Fi access point manufacturers to include location information in their devices. After that, the VoIP operators and the Wi-Fi access operators should be able to sort out some protocol for sharing the location info. Welcome to the 21st century! They never said it was going to be easy. --Michael Dillon