It's an accessibility feature for people losing their voice or on the verge of. And it is TTS only, not speech-to-speech as your mention of "can use it in phone

It's an accessibility feature for people losing their voice or on the verge of. And it is TTS only, not speech-to-speech as your mention of "can use it in phone calls and facetime" implies. Not being s2s means it doesn't retain vocal disfluencies, prosody etc signals that make a voice feel real