HD Calling is High Definition Calling. It is a high definition voice quality for telephone audio. It is also known as Wideband Audio
High Definition Voice calling enables you to make high-quality, crystal clear calls. You can also make and receive video calls with HD-Calling enabled.
Wideband audio, also known as wideband voice or HD voice, is high definition voice quality for telephony audio, contrasted with standard digital telephony “toll quality”. It extends the frequency range of audio signals transmitted over telephone lines, resulting in higher quality speech.Wikipedia
Is My Phone HD Compatible?
How To Enable Free HD Voice of VoLTE on your Android or iphone
What is VoLTE
VoLTE stands for Voice over Long-Term Evolution or Voice over LTE. VoLTE offers the possibility to make voice calls via the 4G network
Voice over Long-Term Evolution is an LTE high-speed wireless communication standard for mobile phones and data terminals, including Internet of things devices and wearables. VoLTE has up to three times more voice and data capacity than older 3G UMTS and up to six times more than 2G GSMWikipedia
What is The Difference Between VoLTE and LTE
The difference between the two networks is that with VoLTE you can send voice and data over the network without losing or affecting the voice quality. On the Other hand, when using LTE your data connection will be on when making a voice call and therefore the quality of voice will be decreased.