On current call, hit transfer, the persons 4-digit extension + #9 and hang up.

Bonus Round....

To answer on another line and forward to someone's voicemail box, you must do the following: (numbers used below are just examples)

Incoming phone call or voice transfer to extension XX11, picked up on extension XX12.  End-caller wants to leave a voicemail in the mailbox of XX11.  

User XX12 must transfer call to their extension XX12 in order to access XX11's mailbox, the process is as follows;

User XX12 transfers to their number (XX12), hangs up their phone so the call transfers, picks up the phone and then asks End-caller to hold for voicemail, User XX12 then transfers again by hitting transfer button, XX11+9 and hangs up.