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 index 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. user_expiry (http://wiki.snom.com/Settings/user_expiry) - The proposed expiry time of the registration in seconds for line x. Upon expiration of the registration, the phone will send a fresh re-registration request.
e. sip_health_check (http://wiki.snom.com/Settings/sip_health_check) - Enables/Disables the status polling of primary server if the phone has been failed over to the backup server
f. session_timer (http://wiki.snom.com/Settings/session_timer) - Session Timer for calls (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)