VoLTE sound quality HORRIBLE on Iphone 7

My parents have had VoLTE switched off since they got their iPhones, because the sound quality was so awful. It seems to pick up every little whisper of breath that they have, even when we put on the background noise suppression thing that I can't remember the name of. Any ideas? We just had to switch them to LTE when 3G shut down yesterday, and it sounds like their voices have both gone up an octave and it's driving me CRAZY on calls.