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.
Yes, you can. You must configure a voip dial-peer and a pots dial-peer with great value of "preference" to remote UC.
This is an example:
dial-peer voice 100 voip
preference 1
destination-pattern REMOTE UC NUMBER
session protocol sipv2
session target ipv4:REMOTE UC IP
... omitted ...
dial-peer voice 101 pots
description POTS_backup
preference 2
destination-pattern REMOTE UC NUMBER
port 2/1
... omitted ...
Commands "preference", "hunt stop", "voice hunt" handle the hunt process on different dial-peers.
Regards.