Recording for voicemail systems

I recently had to pass on a voicemail VO project. I couldn’t get the quality that the
client wanted either in 8 or 16-bit mono. Is there a “best” audio file format setting for voicemail recordings?
If so, are the settings settable on Audacity 1.2.6? Thanks for responding!

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Ummmm. What? What’s a voicemail voice over?

Music CD sound is 16-bit, so something else was going on.

Koz