If you are using failover and a Personal Assistant server becomes disabled, the failover server takes control of the media ports and interceptor ports that were configured on the disabled server. For example, if you configured 15 media ports on the disabled server, the failover server would add 15 media ports to its configuration. Thus, if you use failover servers, you must have twice as many servers for a given number of media ports as would be required if you were not using failover servers.
Although the failover server takes on the media and interceptor ports of the disabled server, it cannot take over active calls. Any active user calls on the disabled server are dropped. However, if Personal Assistant has completed its role in the call process (for example, it had transferred a call based on call-routing rules), the call remains in progress. In addition to taking over the disabled servers ports, the failover server registers itself with Cisco CallManager as the disabled servers CTI route point.
This URL should Helps you:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_callmg/3_3/rel_note/332cmrn.htm
http://www.cisco.com/application/pdf/en/us/guest/products/ps2026/c2001/ccmigration_09186a0080088bdf.pdf
Regarding Typical bandwidth utilization in order to keep alligned the two CRS database, this URl should helps you:
http://www.cisco.com/en/US/products/ps5763/index.html
