The TVA series can be set to look at both first and last names.
Also as with any VM with multiple Greetings per mailbox, be sure to check the Busy and After Hours greetings when users change. This will also confuse callers.
It's usually easier as an Admin to delete the mailbox and make a new one Via 999 and record the name yourself, then all is ready with no fuss. Be sure to put in both names if first and last is on.