Telephones have been using a limited frequency range of 300 hertz to 3.4 Kilohertz for over 100 years. While the frequency spectrum of the human voice ranges from about 50 Hertz to 8 Kilohertz, speech remains quite intelligible when transmitted at the very limited bandwidth. Even the introduction of digital telephone technologies such as ISDN did not change this bandwidth limit. While ISDN does have two parallel, digital voice channels, both channels still only have an analogue bandwidth of 300 Hz to 3.4 kHz.
The introduction of Voice-over-IP telephony (VoIP telephony) and the high transmission capacities of the IP networks and IP lines allow higher frequency spectrums for calls. Wideband audio extends this frequency range to 50 to 7,000 Hertz. This allows deeper and higher speech tones to be transmitted over the phone line.
HD voice calls significantly improve the natural sound and intelligibility of speech. It is much easier for the parties to understand what the other is saying, both in loud surroundings and in quiet conversations. Transmitting virtually the entire spectrum allows for better voice recognition and along with it identifying people. Wideband audio is also beneficial for conference calls where several individuals may be talking at the same time.
Wideband audio calls require the entire chain of technology, from the caller to the person being called, to support wideband audio. This includes both phones, any mobile handsets, the access routers and the transmission network. If one of these components is not HD voice compatible, the call will revert to standard narrowband technology.