The CFA override option located in the System Parameters page on the CUCM Server allows a caller to transfer a call to a phone in which the destination of the configured CFA is the same as the caller’s DN. Bit of tongue twister, below steps it out.

Configuration: Phone A has CFA configured with destination of Phone B.

Scenario 1: If Phone C calls Phone A, the call is forwarded to Phone B.
Scenario 2: If phone B calls Phone A, the call will be successfully delivered to Phone A. Why? This is because Phone A has the CFA destination of Phone B.