I think you are confusing DN (Directory Number) and RN (Received Number).
A DN is a number that identifies a device on the system. A DN can be dialed only by internal parties on internal lines. Users outside the system cannot dial DN.
A RN is a number that identifies a target line. A RN can be dialed only by external parties on auto-answer trunks. Users inside the system cannot dial RN.
A DN can be the same as a RN because they are not used by same party.
When you see "Invalid Number" on the display, it means that the DN doesn't exist. If you look for DN 2345 under Terminal & Sets, you won't find it.
A set cannot be redirected, but it can be forwarded. If the set was forwarded and later unplugged, you would see "Not in Service" on the display.
A line or a target line can be redirected. If you look for a target line with RN 2345 under Lines, you will find it.
If you want a DN 2345, you will need to pick an unused DN (for example, 2399) and change it to 2345.