For example, this is how Nextiva’s call analytics page looks like. You would need to track how many users hear each voicemail greeting and how they responded. Voicemail greetings with a higher score of customers leaving a message are working efficiently. Voicemail messages that are not even heard completely are a failure.
You can play a voice message at different speeds and pause/resume the message, using desktop client visual voicemail, or by dialing into their voicemail. Pressing a certain key on your phone controls how you can hear the voicemail: Read your voicemail transcription

Recording professional voicemail greetings is easy but many people find it to be akin to public speaking, and are therefore apprehensive about starting. First of all, lets acknowledge the elephant in the room and just say that nobody in the history of the world enjoys the sound of their voices being played back to them. Once you can accept that, getting a professional voicemail greeting or Auto Attendant menu recorded will be a lot easier. To help you get the right tone and tenor of what you’re trying to communicate over you business voicemail, we’ve put together this handy list of resources for you.
Informal voicemail greetings are those that don’t adhere to the rules of professional and business voicemail greetings. Here, you are free to just leave a simple greeting, or use it to show your more witty side. This style of greetings can be used for both home and business if you like, depending on the type of caller you expect. For instance, a novelty shop could probably create a rather informal message, using sound effects that represent the type of items they sell.