Redirect to offsite server during downtime

[rbtux]

depends on your setup. Assuming you use bgp4 (as every isp does) you could route a /32 IP to your ispcp box and use it as virtual address. In case one facility goes down, the ip is rerouted to the second box. (For that purpose you configure the prefix at the backupsite with a lower localpref, so when the ibgp session to the router of your mainfacility goes down, the backup route becomes the best valid route...)