A call connection method in an IP multimedia subsystem (IMS) is provided. The method includes the following steps. An entrance network element (NE) of a called network receives a session request carrying called user identification (ID) information from a calling network. When determining that the called user ID information is incomplete, the entrance NE of the called network sends a response message indicating that the called user ID information is incomplete to the calling network. The calling network updates the called user ID information according to the response message, and sends the updated called user ID information to the entrance NE of the called network. An interface NE, a called network system, a call connection system, and a method of informing a call connection failure are also provided.