Go into form 8 and give each LDN a unique number (I normally use 1002 - 1010 if there are no conflicts) . Give each LDN where you are going to direct calls a label (name) LDN is normally reserved for dial 0 calls and I usually label this as IN HOUSE or INTERNAL . Then in form 19 direct the calls to the appropriate LDNs. If you are using form 14 for trunks then you would want to put the appropriate LDN there for "Day" (I usually send night 1 to a multi-button set at the front desk and night 2 to a cordless (if they have one at the front desk) this can change if they are using the embedded auto-attendant.

Just one other thing if you have not set one of these up before. In order for the VM to work properly Dial 0 in form 19 has to go to the same LDN for Day, Night 1, and Night 2. If you need dial 0 to go to different places for those services for the extensions then put priority dial 0 in their COS (option 239) and route appropriately in form 19