How to set a pre-recorded audio file as my outgoing voicemail mesage?

I can’t believe T-Mobile (or Sprint before them) still don’t allow you to set up and use a pre-recorded audio file (e.g. .mp3) as your outgoing voicemail message.

Sure, you can play the file over an external speaker and record it using your phone’s microphone, but the quality is not good.

Is there a reason I'm missing as to why this capability is unavailable on my Android phone? Security? Copyright?