The SIP connection would work through the LAN interface. So the upstream SIP interface ideally should be on the same subnet as the LAN interface, or at least have IP connectivity to it. So if your controller LAN interface is 10.1.10.10 with a 255.255.255.0 subnet mask, have the provider provision their SIP handoff for an available IP in the 10.1.10.x range, connect the SIP connection to a port on the same VLAN as the controller LAN port, and you should be good.