Which circuit-switched WAN technology is often used to provide a backup for a leased line and additional capacity during peak usage times?

The circuit-switched WAN technology often used as a backup for a leased line and to provide additional capacity during peak usage times is ISDN (Integrated Services Digital Network). ISDN can dynamically allocate bandwidth as needed, making it an ideal solution for businesses that require reliable connections and flexibility. It complements leased lines by providing an alternative path for data transmission when the primary line is congested or fails.