HD Voice, also known as wideband audio, is a technology that delivers better comprehension and clarity in person-to-person calls, conference discussions, or a speech-to-text process such as Interactive Voice Response (IVR). Instead of limiting a call frequency to between 300 Hz and 3.4 kHz, a wideband audio call transmits at a range of 50 Hz to 7 kHz, or higher. That's much more in line with the human voice, which transmits audio between 75 Hz and 14 kHz.
HD Voice takes in more audio samples per second (16,000) than a standard call (8,000), allowing us to hear more details in a person's voice during a call. Both phones on the call must support HD Voice and share a carrier who supports HD Voice.