What would cause 2 DKT to stop following System Call forward settings?

CIX670 System CF 1 set to Busy and No answer Active destination is voicemail (4000)

Ext 3097 and 3292 have just stopped forwarding to voicemail. Both of these are programmed the same as 3212 which is working without issue.

For temp I had the client forward from the phone #6041 4000 20 to get the mailbox to answer