A very popular use for VoIP is "toll bypass" where a local PBX might prefer to route calls to a local router, which...
then converts them to VoIP and sends them across a WAN to the remote office's PBX, to avoid long-distance charges. This works great but as always, there are gotchas. One is "what if the WAN is down?" If the PBX knows the WAN is down, it could route the call over a less-preferred path, incurring costs, but not causing a disruption. However, if the PBX doesn't know the WAN is down, once it sends the call to the router, the router must either send the call back to the PBX, which at best uses twice as many lines and at worst causes a routing loop, or the router can simply drop the call. Neither of these options is generally acceptable.
A solution to this problem is for the router to "busyout" a port when it knows it can't connect calls. This behavior tells the PBX that the path is unavailable before the call is sent to the router, while the PBX still has control. At this point, the PBX can choose an alternate path. On a Cisco router, you can configure the command "busyout monitor <interface>" which tells the router to watch the interface you specify (typically the WAN interface) and busyout the voice port when the interface goes down.
There's one sub-gotcha here. Often, the WAN interface uses Frame-Relay. If you've used Frame-Relay much, you probably realize that because the LMI is by definition local, the far end of the circuit can be down and the local interface stays up. In other words, the interface is "up/up" and your PVC looks fine, but you won't pass any traffic because the other side is down. In this case, busyout doesn't do you any good unless you employ another feature: "end-to-end keepalives". This can be configured via a map-class.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.