Radio Frequency Interference (RFI) occurs when a source of RF emission is located in very close proximity to the cable plant or where adequate shielding and bonding of the telephone facilities is not feasible, such as near or at the customer premises.

Metallic noise enters the network on one side of the pair with the return path provided by the other side of the pair. Longitudinal noise enters the network on either one or both sides of the pair; ground is the return path. Both metallic and longitudinal RF interference are attenuated by these UL listed networks.

Audible RF noise may also be created within the telephone set. Circuits and certain components within a telephone set can act as both antenna and receiver (demodulator) for RF emissions from a nearby source. Maximum performance of any RFI suppression network can not be assured in such situations.