How many people would do it? Not many, I fear.
I know many people who don't have landlines because they use cellphones; surely many VOIP users won't have landlines, either.
Maybe the FCC should mandate extremely low-cost 'emergency only' landlines?!?
If you never moved from your original ISP, your VOIP provider would already have your address and be able to provide it to the 911 authorities... and that would be simpler (& cheaper) than having ISPs intercept calls.
How about having TAs that 'remember' their last GPS reading (e.g. in NVRAM)? When in buildings where GPS doesn't work, it would at least remember the position of the entrance to the building. There are situations where the TA would be shielded from GPS for long distances (e.g. subway rides) and, if the user doesn't need to step outside after those trips, this locating system would not work. I leave it as an exercise for the reader to determine whether this is more effective or more harmful than any other proposal (or the status quo.)
Geoffrey Welsh Never leave until tomorrow what can wait until next week.