Hi,
This is dependant on how the phone detects its loss of connection to the CallManager..
Have a look at this extract from:
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/prod_qas0900aecd8028d113.html
Q. How long does it take for IP phones to fall back to the Cisco Unified SRST router?
A. You get an "instant" failover if and only if:
• Failover occurs when the phones have a hot standby TCP socket already open to the failover device (SRST or the second Cisco Unified Communications Manager).
• Failover occurs when the TCP connection is explicitly closed by a TCP FIN, RST, or Internet Control Message Protocol (ICMP) host unreachable, so that the phone is not replying until it times out: The phone sends station keepalive messages to its primary and backup Cisco Unified Communications Manager servers. The phone knows the TCP connection of its backup server is up because of the keepalive messages, and attempts registration when needed. The following fields are recommended default values (though configurable): StationKeepAliveInterval: 30 seconds; and Station2ndKeepAliveInterval: 180 seconds.
Following are the three scenarios of failovers:
• Failover occurs when the Cisco Unified Communications Manager server to which the phone is registered stops working: If the active Cisco Unified Communications Manager is manually stopped, the failover is immediate because the Cisco Unified Communications Manager closes its TCP connections, causing the phone to register with its designated standby (or backup) server immediately.
• Failover occurs when the Cisco Unified Communications Manager process locks up: This scenario is the worst scenario; if it occurs, failover could potentially take 90 seconds because the phone makes three station keepalive attempts spaced 30 seconds apart as default (30 seconds is changeable, however).
• Failover occurs when a TCP failure occurs: TCP failure could be the result of a router or switch going down, or the server itself going into complete failure mode. As soon as the phone sends its first station keepalive message after the TCP connection is down, it sends TCP retries for approximately 20 to 25 seconds. After that, the phone attempts registration with its designated standby (or backup) server.
Cheers,
Tim
