Customise voicemail message based on caller
Voicemail is a carrier feature and currently, there are no carriers as far as I am aware that permit you to store multiple greetings.

Reference: https://discussions.apple.com/thread/3841850?answerId=3841850021#3841850021


  Voicemail is a carrier feature &amp; your greeting is stored on their servers. I'm not aware of any carrier, anywhere, that permits multiple greetings.