Call forwarding(CF) is a feature of PBXs and exchanges. As can be seen from the term call forwarding, calls to specific subscriber numbers are diverted to other numbers, for example, if the number is busy or the subscriber wishes to receive his calls at another extension.
Call forwarding can be unconditional, i.e. it can be performed immediately without any specific conditions. In this case, it is a Call Forwarding Unconditional( CFU), it can be switched according to predefined conditions, for example, after a certain number of rings. This mode of operation is called Call Forwarding No Reply( CFNR). And as a further option, call forwarding can only be triggered when the line is busy. This operating mode is called Call Forwarding If Busy( CFB).
There is also Selective Call Forwarding( SCF), in which only certain, previously selected calls, for example from mobile networks, are forwarded.