To check your voicemail messages from another phone: Call your 10-digit wireless number. When you hear your voicemail greeting, press the * key to interrupt it. If you reach the main voicemail system greeting, enter your 10-digit wireless phone number, then interrupt your greeting by pressing the * key. Enter your voicemail password when prompted.
For the users to be able to do this from the phone or by calling into the system the users will have to have pre-recorded greeting for different status and switch between them from the phone or by calling in to the system. They can do that from the web client, windows client or an android client.

In the Voicemail section, tap Voicemail greeting. Tap Record a greeting. Tap Record . Record your greeting and when you’re done, tap Stop . Choose what you want to do with the recording: To listen to the recording, tap Play . To replace the recording, tap Redo. To save the recording, tap Save. Enter a name for the new greeting and tap Save.
2. Once there, tap on "Greeting" in the top-left corner. Your iPhone will take a few moments to fetch and load your current voicemail greeting, which you can play using the controls in the middle of the page.