How do I see the number for an unknown caller who is not in my contact list?
Whether the caller's number shown or not does not depend on your contact list. It is the information which is provided by the carrier (AND the caller because the caller can disable it and you won't see any number even if the caller is in your contact list).
If you see "unknown" that means the CallerID was not provided. If it's provided then you would see the number of the callers even if they're not in your contact list.