Hi,
You can use the command "preference" to specify backup destinations with dial peers. Example:
dial-peer voice 2000 voip
preference 1
destination-pattern 2000
session target ipv4:10.1.1.3
dial-peer voice 2001 voip
preference 2
destination-pattern 2000
session target ipv4:10.2.2.3
Lower preference is prefered. Have a look at the command reference for further details, f.e.:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a00800b350f.html#wp999679
In general it might be useful to have a look at the Solution Reference Network Design guides for voice, which can be found at www.cisco.com/go/srnd
It contains design guides with best practice examples for various scenarios.
Hope this helps! Please use the rating system.
Regards, Martin