From the Skype for Business desktop app (if you are running Click-2-Run build C2R 8201.1002 or later), select Set Up Voice Mail. Change your Call Answering Rules You can change what options your callers have when they reach the voicemail service; get disconnected, hear your greeting, record a message, choose to be transferred to the number or user you select. Change your prompt language This is the prompt language that is played to callers. For list of supported languages, see Languages for voicemail greetings and messages from Skype for Business. Configure your Out of Office greeting This is the customized greeting message that is played to callers when your status is Out of Office. This option can be activated "Always," when you have an active "Auto Reply" in Outlook, or whenever you have an out-of-office calendar appointment.  Where you can get your voicemail messages from
Some cell phone service providers allow users to access their voicemail accounts via external methods such as landlines, and those users could keep the voicemail greetings updated in the absence of cellular and internet service.

If you’ve already created a custom voicemail on your iPhone, you can tap Play to see how your voicemail sounds. If you’re interested in creating a new message, tap the Custom button. From here, you can start recording your message by tapping Record and tapping Stop whenever you’re finished. Once you’re finished recording, tap on Play to make sure the message comes across as you’d like and hit Save (in the upper right) to save it as your voicemail.
How to Change the Outgoing Voicemail Message on IPhone (iOS 9 or later) Tap the phone icon (the same one you use to make a call). Tap the voicemail icon in the lower right of the screen (it’s the one that looks like an old-fashioned cassette tape). You’ll be taken to a screen that lists all of your recent voicemails; tap on the word