Core Issue
IP phones continue to send keep alives to the Cisco CallManager every 30 seconds. When the link goes down, the keep alives are missed. Once there are three consecutive keep alive misses, the IP phone un-registers with the Cisco CallManager and the phones register with the Survivable Remote Site Telephony (SRST) router. Since it is set to three consecutive keep alive misses, it takes 90 seconds for the phone to register with the SRST router.
Resolution
During SRST failover, the phone unregisters with any or all CallManagers which hold the call forward destinations. All call forwarding information is lost if a phone loses the connection to the CallManager it is registered to and registers with SRST.
Incoming calls that reach a busy signal or go unanswered during Cisco CallManager fallback can be configured to forward to one or more E.164 numbers.
