I have a customer that has 3 sites. I'm looking at a UC 560 for the main site and UC 540 for the other 2 sites. When a call comes to the UC 560 and all 3 lines at the main site are busy, can the call be routed via the PSTN to a 2nd site, if there are lines free at the 2nd site? Or do they need to have a WAN in place connecting the 3 sites. Currently they don't have a WAN.