I'd inspect these areas:
Recall destinations of HGs...follow the path, don't just accept what's listed (could be manual forwards etc you can't see), so make test calls
Announcement and overflow CRAs, maybe a digit translation that user stumbles into by desperation button pushing...again, follow the path
Kill the extension concerned, if it's always the same one, by removing the ext entirely, then re-create...if there was anything hidden that should cancel it.