The call forward setting for this extension was canceled. In order to reset it you will need to figure out what the voice mails pilot number is, this will be a hit and miss, I used to set it as 300.
From any extension press the speaker key and dial 300, if prompted for a security code 300 will be the pilot number, if you get the reorder tone try another numbers, 200, 250, 350 and so on. Once you have the voice mail pilot go to the phone with the issue.
Press the speaker key dial 43, dial the voice mail pilot number, press the speaker key to hang up, make a test call.

If you are unable to determine what the voice mail pilot number is you will need to call in a tech. They will be able to log into the systems main programming to get this information.

We get old too soon, smart too late