As of iOS17, it would appear that responsibility for synthesizing the Default Greeting preview (locally on one’s device) has now fallen to whichever Siri voice you currently have selected in settings at the time.
Upon noticing this, I verified that the actual recording a missed caller will hear is the same (apparently carrier-side) as its been since before the trees and rocks on Earth and it occurred to me just how much better stock, out of the box Siri Voice 2 sounds than her, so I gave it a shot.
I plan to leave it as my greeting for the time being so - if you’re curious/require hearing what your missed callers will actually hear - horrendous noise and all - I would encourage you to call me, anytime). I do not experience or follow up with unknown callers unless they leave a voicemail… which I’d also be more than fine with you doing.
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Enjoy!