1. Make sure you are using the most current stable release of firmware for your Snom phone.

2. Make sure the Identities are configured and can register with the Registrar.

3. Configure the following settings accordingly;

a. user_failover_identity (http://wiki.snom.com/Settings/user_failover_identity) - The idex of the failover Identity 

b. retry_after_failed_register (http://wiki.snom.com/Settings/retry_after_failed_register) -  Number of seconds after a failed registration the Primary will try to re-register

c. active_line (http://wiki.snom.com/Settings/active_line) - The Primary Identity

d. session_timer (http://wiki.snom.com/Settings/session_timer) - The time the Identities will re-register with the Registrar (try setting to 1800 seconds)

4. Be sure not to use Identity 1 as the Primary with Identity 12 as the failover Identity. There is a known issue (SCPP-6267 - 
Failover not working with Identity 1 and 12
5. Re-try and test the failover.