If you have both FXO trunk and SIP trunk, you will see them in Outbound Trunks page of the configure utility. You can select them in the order, in which you check the boxes is the order in which the trunks are used for the outbound calls. For example if you select a FXO trunk first and a SIP provider second, the SIP provider is used only when the FXO trunk is busy or down.
There is no failover between SIP and FXO for the inbound calls since inbound phone numbers for FXO and SIP trunk are usually different, and they are routed to a sepecfic destination in the Inbound Calls page. The failover is out of control of UC320W. You may want to check with your service provider.