There are several ways you could do this. I had a request for something similar to this myself. In my case the user had an existing extension and voicemail, if the call was unanswered then we still needed the call to go to her voicemail. Here is what we did (by we i mean myself and my Mitel technician):
Lets say the users original extension was 1000, her new extension is 1001.
Voicemail Application: 3000
Objective: have both phones ring at the same time when she receives a call.
1): Create an extension list containing both extensions (System\Devices and Feature codes\IP/Digital Telephone. Right click in the grey area in the right pane and create IP/Digital telephone extension list, assign any number you want.). For this instructional I will say I numbered my list P0030.
2): Select the new extension list in the left pane then right click in the right pane and add the extensions 1000 and 1001 to this extension list. (using lists is my preferred method, it just makes thing easier to manage much like dealing with security groups in Active Directory.).
3): Create a new hunt group (System\Devices and Feature Codes\Hunt Groups\Local(or whatever node this users extensions are on) right click in the right pane in a grey area and click create hunt group, assign any number you want.). For this instructional I will say I numbered mine 2000.
4): Select the new hunt group in the left pane and expand it down, Select the "members" item then right click in the grey area in the right pane and click add to members list... You will then navigate select IP/Digital Telephone extension list and add your newly created list containing the 2 extensions (P0030).
5): Pick an open Forwarding Path (in my case i will use 6) in DB programming (System\Phone-Related Information\System Forwarding paths). put in a meaningful description for yourself. Input the newly created hunt group number (2000) into Forwarding Port 1. Place the Voicemail application number (3000) in the forwarding port 2.
6: Almost home free! I then selected the station 1000 and opened the forwarding paths. I removed her other forwarding path for voicemail, then added forwarding path number 6. Double click the 6 under the stations forwarding path and then set the value for "Station - Immediate" to yes.
You may want to check your System\Phone-Related Information\Timers and Limits and make sure System Forward Advance and System Forward Initiate. are set at matching times, this will control when the system pushes the call to the next forwarding point (in my case the users voicemail box.). BEWARE that modifying these timers can affect other things in your system, just know what you're changing before you change it, Also Mitel has an awesome feature where you can press F1 while in DB Programming and bring up help files for the current screen.
DISCLAIMER: I hope this write up helps you out. I claim no responsibility for any actions you take with your phone system, actions you take from my write up are the responsibility of yourself alone. I highly advise that if you are not comfortable making changes, contact Mitel and put in a request for some help with programing, they're always a great help.
Have a great day,
- Michael