When you try to get your voicemail, does your phone dial an extension?
What caused this? Someone from the extension your phone is trying to call tried to call you, then activated the "Call Back" feature.
How do I fix this? You have two options:
Option #1: When you you try to get your voicemail and the extension rings, have someone pick up that extension.
Option #2: On your phone select Menu > Feature Access > Call Back Cancel (Enter)
If you have tried both options and neither work, please create an IT Ticket.