LINE Planet's audio call quality is FHD according to the specification. What is FHD?

Audio call quality is typically measured in terms of the frequency range that can be represented. The higher the representable frequency range, the cleaner and clearer the sound.

Speech is mostly in the lower frequency band, and speech signal processing modules take advantage of this by discarding or not considering higher frequency components in their processing. This type of speech signal processing reduces the bitrate, but can degrade the user's perceived quality. The lower the representable frequency range, the more muffled or stuffy the sound.

Audio call quality is classified as narrowband (NB), high-definition (HD), and full high-definition (FHD). NB is the lowest quality and FHD is the highest quality. For more information, see audio quality.

For your reference, most carriers provide HD quality voice calls on LTE networks.