CISCO Unity: External Calls Go Directly to Voicemail

A user's external calls go directly to voicemail without ringing the extension while internal calls ring normally.

Deleting and recreating the extension does not resolve the issue.

Deleting and recreating the phone entry does not resolve the issue.

Another phone set configured to the same extension has the same issue.

Delete and recreate the user's voice mailbox in Cisco Unity connection.
Issue does not reoccur.