US Patent 7323973 Multiplexed TTY signaling for telematics

In response to a triggering event at a location such as a vehicle that has crashed a telematics system initiates a telecommunications call via telecommunications link 8 to PTN 10, through which it is routed to local 9-1-1 system 14 and thereby to local PSAP 18. Telematics system 2 waits, 26 (N), for local PSAP 18 to respond, 26 (Y), then exchanges ACN data with it per TTY protocols, 28. The PSAP then sends a command code back to the vehicle or other location to allow for voice communications with a person at the location. Other functions such as shutting the vehicle engine off can also be controlled by PSAP.