I have had great luck using alternate extensions for that exact purpose.
If you have issues determining what digits are being sent to Unity Connection by ARC, you can use the Port Status Monitor to view the calls to the VM in real time.
Available at the following link:
http://ciscounitytools.com/CUC_2xTools.htm
One caveat- The extensions cannot be searched, so if, down the road, someone wanted to add a mailbox with that extension, for example, they will have to know to find the mailbox and remove the alternate extension.