Use Professionally Recorded Sound File for Outgoing Voicemail Greeting? (MP3 or WAV)

I purchased a separate iPhone to serve as the primary phone for my small business and hired a voice actor on Fivver.com to record a professional voicemail greeting for the business (a service that is quite popular, BTW). I can save the MP3 file directly to my iPhone but there's no way to use it for an outgoing voicemail greeting.

Apparently, the only to use a recording for an iPhone's voicemail greeting is to play the recording on one device and record the audio with the iPhone itself. This results in a very poor quality voicemail greeting that sounds like an answering machine from the 1980's. It is unprofessional and unacceptable for a serious business.

Why does Apple not allow iPhones to use a small sound file for the outgoing voicemail greeting?