It would be quite useful in a general sense to develop a technique to transfer modest amounts of data over a voice call - location, obviously for 911, but also for plenty of other applications where installing an app (never mind developing it) is overkill. Such a technology could also be used for automatically transferring reservation/membership numbers when calling customer service, or even enabling some kind of secure challenge/response for calling your bank.
I suppose it could work with some kind of low-speed acoustic modem combined with an "intents"-style protocol. You'd hear a short sequence of beeps, your phone would vibrate and show a dialog "Caller wants to know your GPS location", and when clicking OK, your phone would send back your location. In the other direction, when initiating a call with metadata, the receiving party would play a beep when ready (like a fax) and your phone would play off the metadata for the call -- this would basically be implemented like query string parameters on the phone number URL.
I suppose it could work with some kind of low-speed acoustic modem combined with an "intents"-style protocol. You'd hear a short sequence of beeps, your phone would vibrate and show a dialog "Caller wants to know your GPS location", and when clicking OK, your phone would send back your location. In the other direction, when initiating a call with metadata, the receiving party would play a beep when ready (like a fax) and your phone would play off the metadata for the call -- this would basically be implemented like query string parameters on the phone number URL.