A cage valve, flow trim components, and method for signalling erosion of flow trim. The valve is of the type having flow trim arranged across an inlet bore and aligned with an outlet bore, and including a stationary tubular cage and an internal or external flow control member such as a plug or flow collar slidable along the side wall of the cage to cover or uncover flow ports in the side wall. The flow control member includes an end plate which closes an upstream end of the cage. A cavity is formed in the flow control member upstream of the end plate. A transmitter, such as a proximity beacon, positioned in the cavity transmits a first signal indicative of intact flow trim when no fluid is in the beacon cavity and transmits a zero value signal or no signal indicative of eroded flow trim when fluid enters the cavity.