These issues limit the quality of the recording because the amateur actor doesn’t know how to speak with the proper enunciation and cadence for phone system recordings, and there’s no experienced director handy to provide guidance.
First you need to know how much time your phone system or provider allows for the duration of the greeting. Some are 30 sec some are longer. Then you need to have a few ideas of what you want to say, or ask your callers.

Your voice conveys more than you may realize. We can sound bored, excited, tired, sick, relaxed, or happy just by the posture we have when we’re on a call. It’s true that when you leave a voicemail, no one will see you slumped in a chair, but they will be able to hear it. Don’t overdo it, and don’t be fake; that comes across in a message, too. Again, leave a message that sounds like it would if you were talking to someone in person. Smile, sit up straight, stand if you prefer, and feel free to use any gestures you would typically use.
Some phone systems allow employees to specify a primary greeting and set secondary messages to cover absences or vacations. When creating an individual voicemail greeting, clearly identify yourself, your role, and your company, and let the caller know when you will be able to return their call.