Various audio codecs have the same thing. They will synthesize background noise. The amount of background noise and the rough frequency spectrum are encoded in

Various audio codecs have the same thing. They will synthesize background noise. The amount of background noise and the rough frequency spectrum are encoded in the stream. If you removed the noise, it would sound a bit weird. For telephone calls in particular, the background noise lets you know that the phone call is connected. If you filter it out, people think that the call was dropped.