Routing traffic to a backup webserver

We run several websites using our own webservers and leased line at our main office. We’ve recently opened a second office with a second leased line and would like to locate our backup webserver there. Is it possible to configure our setup so thatif our main webserver is unavailable traffic is automatically routed to our backup webserver, and if so, how? I’ve spoken to our ISP and they can’t offer a solution.