The only justification is the ease of communication over the phone.
I only don't like that it's proprietary and the company responsible is pretty aggressive about enforcing their rights - there shouldn't be a place for that in public emergency services.
I mean.. as opposed to two numbers? A slow speed high error correction audio encoding of them would be easy. Call emergency services, press the button, they decode the tones, immediate, accurate, simple.
If you mixed the calling and playback into a single action, you could make calls and direct people without even the ability to speak.
Maybe it helps with an issue they have noticed with certain callers that I'm not aware of.