This is what SRND Guide of 7.5 says
When the active CTI Manager Service or PG software fails, the PG JTAPI Gateway/PIM detects an OUT_OF_SERVICE event and induces a failover to the redundant (duplex) PG. Because the redundant PG is logged into the backup Unified CM subscriber CTI Manager Service already, it registers the IP phones and configured dialed numbers or CTI route points automatically. This initialization service takes place at a rate of about 5 devices per second. The agent desktops show them as being logged out or not ready, and a message displays stating that their routing client or peripheral (Unified CM) has gone off-line. (This warning can be turned on or off, depending on the administrator's preference.) All agents and supervisors lose their desktop third-party call control functionality until the failure recovery is complete. The agents and supervisors can recognize this event because call control action buttons on the desktop will gray out, and they will not be able to do anything with the desktop. Any existing calls should remain active without any impact to the caller.
Regards
