If she has an actual telephone set then it is easiest for you to just set up the phone and externally call forward the phone when she's out. (see the FAQ on how to externally call foward a phone)
If she does not have a telephone set then you can give her a guest mailbox and then assign her DID to it by doing the following:
TELECOMMUTING GUEST MAILBOX SCENARIO
SCENARIO: A salesperson is often on the road and has no extension at the home office. The sales person needs to receive business call messages and will be given a DID line if it is possible to program with a mailbox. This system is a Norstar® MICS 4.1 with a NAM II voicemail 4.0.
Line Programming
1. Build a Target line, example: 145
2. Enter customer received digits, example: 4598
3. Prime Set: None
Hunt Group Programming
4. Select a Hunt Group number (1-30)
5. Accept the default Hunt Group Extension that is associated with the Hunt Group number or make a new one.
6. Add a Member DN to be that of the NAM Voicemail DN.
7. Under Line Assignment, assign the Target Line to the Hunt Group.
8. Shorten the Time Out Que to be the lowest, 15 seconds (default = 60)
9. Make the Overflow extension to be the NAM Voicemail DN.
Mailbox Programming
10. ADD a guest mailbox
11. Make the Extension of the mailbox to be the Hunt Group Extension.
That should do it for you,
MRoberts