To transfer a call to someone's voicemail, without ringing their phone.

1. answer the call (obviously)
2. press the flash/tap/transfer button
3. dial 6789
4. press * when voicemail answers
5. dial the extension number of the person to receive the message
6. hang up immediately, so that the caller hears the complete greeting

This utilizes the quick message function of voicemail.
Refer to the top of the Quick Reference Guide, for quick message.