If you don’t know what to say in your voicemail, here’s a few pointers to get you started: Make sure that you’re not yelling into the phone and the message is recorded in a somewhat quiet area. If the message is for work, try to keep it professional and avoid mistakes, if possible. If you’re working in retail, apologize for not picking up and note that you will call them back. Regardless of your occupation, try to keep the message short. It shouldn’t be longer than 25 seconds. If the voicemail is personal, feel free to leave a silly, short message that lets the caller know it’s you.
Yes, though if there's no preexisting greeting the GUI won't detect it if placed in the filesystem. I suspect a database entry is needed for the GUI to "see" the greeting. As a workaround a greeting could be recorded (so that the database entry is present) and the file replaced, but that's getting to be quite a task end-to-end. Also I don't know if the database stores the length of the recordings, which could be problematic during playback if the original and replacement audio files are different lengths. I will do some testing. Thanks. Aug 16, 2018 #4

If you’re less than satisfied in your current phone system voicemail greetings, there is probably a decent reason why. We only employ professional voice talents because we want you provide you a highly effective and high quality recording.
With visual voicemail you may not remember how to call voicemail to set it up. We’ll show you how to record a new iPhone voicemail message and listen to your current one.