Specific number is receiving calls from an unassigned number instead of our main line.

Greetings,

My boss is receiving phone calls and voicemails from a line that is unassigned to any of our users. I was assigned previously until the user was off boarded. Instead of the calls to that line being redirected to our call center lines or main line it is going to my boss. I apologize if this as already been addressed, if so can I be forwarded to the article? Thank you.