Snom phones support two failover methods:
Registration is fine on all accounts.
Primary registrar server goes down.
Calling a number via primary failes.
Phone has no idea about the failed server until next reregistration.
It marks the account as unregistered.
Calling a number via primary detects that the primary is down (regstate!) and set the failover identity as current identity. for the call.
Primary registrar goes up again.
Phone will route outgoing calls via secondary server until primary reregistration succeeds again.
You can configure the failover identity via the setting Failover Identity.