Hello,
with the BACKUP INTERFACE command, the backup interface goes up when the primary interface goes down, independent on any (dynamic) rotuing protocol. Let´s say your primary line is serial 0 and your backup line is BRI0, your config would look like this:
interface serial 0
backup interface bri0
Check thsi link for more info, also on how to use the dial backup with dialer profiles:
Dial-Backup Configuration
http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1831/products_configuration_guide_book09186a00800ca4ea.html
HTH,
Georg