A network interface circuit that screens out transient signals and provides an indication to the line side that an actual event has occurred, so that appropriate discrimination circuitry is powered up to determine the exact nature of the actual event only when actually needed. The present invention develops an AC signal that represents the events that are desired to be detected. This AC signal is timed so that it has a sustained rate (e.g., a 1 millisecond burst) that is unlike any transient that would occur on the line. Thus, unless the incoming signal meets the timing requirements of the circuit, it is disregarded as a being a transient and no action is taken to determine the exact nature of the signal. This avoids the need to invoke the discrimination circuits that have large power requirements until they are actually needed.