The phones should fail back automatically... the resilient controller will check the status of the primary on 60 second intervals after 5 good checks (by default) the phones are release back the primary ICP, this is set in System Properties, System Settings, Controller Registry, using the options:
Resiliency - Allow Return to Primary ICP (Yes/No) - yes
Resiliency - Secondary ICP Health Check Interval (seconds) - 60
Resiliency - Successful Health Checks Prior to Hand Off (1 - 20) - 5