The description of this VAS is located in section Call transfer.
In case if PBX is configured according to instruction Configuring PBX, call transfer works out of the box without additional configuring.
However, call transfer could be initiated only by Users.
Therefore, on incoming call with routing Originator -> User and Gateway -> User, transfer could be initiated only by callee.
When calling User -> Terminator or User -> Gateway, transfer could be initiated only by caller.
When calling User -> User, transfer could be initiated by any peer.