Interesting that article doesn't mention G729 (http://en.wikipedia.org/wiki/G.729) which has VAD (voice activity detection), but also specifies a noise coefficient. A voice frame contains 10 bytes of data, where a noise frame contains 2 bytes. The two byte frame contains a characteristic for the noise, so the other end hears the equivalent (roughly) noise. Cisco also implemented the same thing in G711, but its proprietary.