Hi David,
The UC320W doesn't have a general ARS feature. However, the UC320W Configuration utility gives you the option to specify which trunks (FXO or Sip) to route your emergency calls to.
Look under menu option Ports and Trunks - Outbound Trunks - Emergency Calls.
Further, you can use trunk access codes to determine which trunk type to select. For example, you can assign 8 to select you Sip trunks and 9 to select your FXO trunks (there are other variations that are possible that allow you to group your trunks with same or different access codes).
Hope this helps,
Jeff