Voicemails don’t necessarily have to be monotonous or extremely jazzy or funky. What you should be looking for is a balance between the two. Whenever you call someone and their voicemail greeting is playing, it tells you a lot about who they are, as a person. Which is why, you’ve got to put a little bit of “you” when it comes to voicemail. And the best part about all of this is, since you couldn’t come to the phone, at least the callers are having a good time listening to the message.
If your voicemail has been set up and you are unable to change your greeting through the 'Voicemail' tab, dial *86 then choose option 4 to change. From a Home screen, tap the Phone app . If an app isn't available on your Home screen, swipe left to access the App …

You would think there'd be app for that, but Apple doesn't allow third-party apps to access the microphone and the integrated Phone app directly. However, there are tricks within some apps and services to get around that limitation that can make you a mobile wire-tapping master.
This iOS app does basically the same thing as the above method. It starts a call, initiates a second call, then merges the two so it can be recorded. However, instead of calls being stored in your voicemail, they’re stored in the app. Automatic Call Recorder Pro costs $4.99.