SE450: Analyze Use Case: Leave a message [60/76]

1. User dials extension. Telephone sends number to Connection
   (Add collaborator Telephone to Connection)
2. Connection asks MailSystem to find matching Mailbox
3. Connection asks Mailbox for greeting
   (Add responsibility "manage greeting" to Mailbox,
   add collaborator Mailbox to Connection)
4. Connection asks Telephone to play greeting
5. User speaks greeting. Telephone asks Connection to record it.
   (Add responsibility "record voice input" to Connection)
6. User hangs up. Telephone notifies Connection.
7. Connection constructs Message
   (Add card for Message class,
   add collaborator  Message to Connection)
8. Connection adds Message to Mailbox