Suppose a new message forwarding feature is added to the voice mail system, so that the main menu has a new option, Turn on Forwarding. This option allows a mailbox owner to select an alternate mailbox to receive messages.

Write a complete Turn On Forwarding use case.

Turn on Forwarding

1. Mailbox owner carries out Log In
2. Mailbox owner selects "Turn On Forwarding" menu option
3. Voice mail system speaks:

     Enter forwarding mailbox number followed by #

4. Mailbox owner types in extension number.
5. Mailbox owner presses "#" key.
6. Voice mail system sets the forwarding mailbox number.

Write a variation on the Turn On Forwarding use case.

Variation #1. Invalid forwarding mailbox

1.1 Start at step 4.
1.2 Mailbox owner types an invalid extension number
1.3 Mailbox owner presses "#" key.
1.4 Voice mail system speaks: 

      Invalid extension number

1.5 Continue with step 3

Another possible variation: hanging up before confirmation.