Express Message is used where callers are prompted to enter the number or the name of the person they wish to reach. The caller is answerer with "Express Messaging to mailbox? Pls enter the mailbox # or the name. To enter a name, press (usually 11) spell the last name, then spell the first name. This is also the DN used when transferring a caller to somone's mailbox.
If you are asking about name dialing only, this is usually a thru dial selection in a voice menu. ie: Press 1 for the company directory, where "1" is thru dial by name. The caller is then prompted to spell the name of the person they wish to reach, then they are transferred to that DN.