If it's a system wide issue, you look at the setting in program 20-02-12 which should be checked for all phones to default to forced intercom ringing.
This feature can be changed to voice announce on a station by station basis, and if it's just the one extension, that's probably what happened here - by accident.
Assuming the service code has not been changed from the default - just go to the extension that is not ringing on intercom, lift the handset, dial 823 and hang up. That should switch that extension back to intercom ringing.
