Your department will have its own voicemail box. Those with access to that voicemail will have a separate button on their phone, typically labeled General VM.
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To set up voice mailvoicemail:
Pick up the line you want to set up (General VM).
Press the voice mail voicemail button
Press 269266# (default PIN) on the keypad.
At the prompt, choose option 3 for voice mail voicemail greetings
When you set up the greeting, press *1 to return to the main menu
Press 0431 on your keypad to change the PIN on the line you're configuring. This PIN should be shared with all members of the department that will have access.
When you make the change, hang up the phone.