It's hit and miss whether you can get an extn to twin a Hunt Group call.
I did get it to work for someone who requested it on Tek-Tips but when I tried it again it didn't work
My post from over there:
Create a Phantom
Create a user for the Phantom
Setup DEE for the user, set to yes, add mobile number etc.
Create a Call Routing Application
Set the time out destination as the Phantom extn number
Enter the CRA number into the Hunt Group as a member and calls will twin with the Phantom and the mobile.
This worked and I was happy
I couldn't stop tinkering though so I then stuck the Phantom into a Hunt Group Member list, deleted the CRA from the hunt group and entered the Hunt Group Member list, this worked as well, ringing the mobile when a call was routed to the HG. Theoretically as long as you had enough lines you could ring loads of mobiles.
The full thread:
http://www.tek-tips.com/viewthread.cfm?qid=1700562