> transmit your location during a phone call
Transmit how? Send an SMS to 911? Some kind of HTTP/API call? Some kind of encoded audio signal?
[1]: https://en.wikipedia.org/wiki/Signalling_System_No._7
[2]: https://www.wired.com/2017/05/fix-ss7-two-factor-authenticat...
The more I think about it, having your phone send an SMS to 911 with your location as soon as you dial 911 seems really simple. SMS to 911, where supported, must already be special-cased. Carriers can each deal with that and communicate that back to the 911 call center in their own way (obviously negotiated with the 911 agencies) and phone manufacturers need only do a single (simple) thing.
There would be a lot of flexibility there too. Your phone could choose to update the location as it changed (moving vehicle) or as precision increased.
But... The 911 operator has to initiate the SMS exchange. Kind of strange. I believe that, in certain jurisdictions, TDD service for the deaf was a revenue stream for the incumbent telcom carrier. So, cell phones are specified to have to be "registered" to allow 911 SMS (this is just my speculation). The 911 operator has to initiate the exchange (in the areas that the system I work on is used).
Incoming 911 data already contains location data (from tower triangulation, etc.). More interesting is that the 911 operator must manually request location updates (secondary reports) and that these cannot be requested more frequently than 35 seconds apart (again, these regulations apply in my area). Again, stinking regulations.
Anyway, the data available (formats) are documented by NENA (National Emergency Number Association). www.nena.org. But... carriers (Bell, Telus here) use supersets of those formats.
The problem is not flexibility -- the systems are in place, and must inter-operate. IP networking for these systems is not completely in place (some use serial communications). The cut-over to IP networking is still on-going, and some places use parallel serial and IP networking.
And, of course, 911 operators get ANI/ALI (automatic number identification, automatic location identification) data for home phones (ALI by database, usually), and the mobile needs to be incorporated in such a way as not to confuse the dispatch operators.
I believe the SMS facility is actually built on top of SS7 anyway though, so that actually makes sense. And it wouldn't mean exposing anything new up to the phone level. Not sure what the state of security is though. How hard is it to spoof an SMS message?